Microscope objective lens

A microscope objective lens of medium or high magnification having a great working distance and yet having an excellent imaging performance comprises, in succession from the object side, a first lens group of positive refractive power for converting a light flux from an object into a convergent light flux, the first lens group having a positive meniscus lens component having its concave surface facing the object side and a cemented lens component, a second lens group having a cemented lens component of small refractive power disposed in the convergent light flux, and a third lens group having a meniscus lens component having its convex surface facing the object side and a succeeding negative lens component. The objective lens satisfies the following conditions: EQU .vertline.r.sub.1 .vertline.>.vertline.r.sub.2 .vertline.>f EQU 3f>1/2.vertline.r.sub.1 .vertline.>d.sub.1 EQU d.sub.F +d.sub.A >d.sub.R where r.sub.1 and r.sub.2 are the radii of curvature of the object side and image side lens surfaces, respectively, of the positive meniscus lens component in the first lens group which is most adjacent to the object side, d.sub.1 is the center thickness of the positive meniscus lens component, f is the focal length of the entire system, d.sub.F is the center thickness of the meniscus lens component in the third lens group which has its convex surface facing the object side, d.sub.R is the center thickness of the negative lens component, and d.sub.A is the air space between the meniscus lens component and the negative lens component.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a dry system microscope objective lens of medium 
or high magnification. 
2. Description of the Prior Art 
Heretofore, in microscope objective lenses of medium or high magnification, 
the working distance has generally been small and for example, in an 
objective lens of the order of forty times, the working distance has been 
0.2-0.5 mm, and in an objective lens of the order of one hundred times, 
the working distance has been 0.1-0.3 mm. In objective lenses of such 
small working distances, the fore end thereof has been liable to touch an 
object to be examined during the operation of the microscope to injure the 
object to be examined and also, an inconvenient situation has been liable 
to occur during operation. Therefore, objective lenses having a high 
magnification and yet having a great working distance have been desired, 
but maintenance of the planarity of the image plane and correction of 
aberrations have been more difficult as the magnification becomes higher. 
Also, generally, microscope objective lenses used under transmission 
illumination like those for biological purposes are designed on the 
premise that the thickness of the cover glass is a predetermined reference 
value and therefore, where the thickness of the cover glass differs from 
the reference value, the imaging performance of the objective lens is 
deteriorated. Such tendency becomes more remarkable as the N.A. (numerical 
aperture) of the objective lens is greater. For this reason, as an 
objective lens with a correction ring, there is known an objective lens in 
which the lens spacing in the objective lens is varied with a variation in 
the thickness of the cover glass to thereby prevent aggravation of 
aberrations and maintain a substantially good imaging performance. 
However, in the conventional popular objective lens with a correction 
ring, the range of aberration correction for the variation in the 
thickness of the cover glass is very narrow, and in the case of N.A. of 
the order of 0.6, the thickness range of 0.2-0.3 mm has been the practical 
limit. 
In contrast, Japanese Laid-open Patent Application No. 142508/1981 
(corresponding U.S. Pat. No. 4,403,835) discloses a technique in which a 
microscope objective lens comprises, in succession from the object side, a 
first lens group which is a positive cemented meniscus lens having its 
concave surface facing the object side, a second lens group which is a 
positive lens or a cemented positive lens and a third lens group of 
positive synthesized refractive power and wherein only the second lens 
group is moved along the optical axis in accordance with a variation in 
the thickness of a parallel flat plate disposed between the object surface 
and the objective lens, whereby a good imaging performance is maintained 
even if there is a wide range of variation in the thickness of the 
parallel flat plate. According to this technique, it is certainly possible 
to maintain an excellent imaging performance over a very wide range of 
variation in the thickness of the parallel flat plate, say, .+-.1.0 mm. In 
this technique, however, an objective lens having N.A. of the order of 0.6 
and a magnification of the order of 40 times is the practical limit, and 
this has been insufficient as an objective lens having a greater N.A. or a 
higher magnification. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a microscope objective 
lens of medium or high magnification having a great working distance and 
yet having an excellent imaging performance. 
It is a further object of the present invention to provide a microscope 
objective lens which, in spite of having a great numerical aperture and a 
high magnification, can always maintain an excellent imaging performance 
even if the thickness of a parallel flat plate such as a cover glass 
disposed between the object surface and the objective lens is greatly 
varied. 
The invention will become fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A microscope objective lens according to the present invention basically 
has, in succession from the object O side, a first lens group G1 of 
positive refractive power for converting the light flux from an object 
into a convergent light flux, the first lens group G1 having a positive 
meniscus lens component having its concave surface facing the object side 
and a cemented lens component, a second lens group G2 having a cemented 
lens component of small refractive power disposed in the convergent light 
flux, and a third lens group G3 of negative refractive power having a 
meniscus lens component having its convex surface facing the object side 
and a succeeding negative lens component. 
The microscope objective lens satisfies the following conditions: 
EQU .vertline.r.sub.1 .vertline.&gt;.vertline.r.sub.2 .vertline.&gt;f (1) 
3f&gt;1/2.vertline.r.sub.1 .vertline.&gt;d.sub.1 (2) 
EQU d.sub.F +d.sub.A &gt;d.sub.R (3) 
where r.sub.1 and r.sub.2 are the radii of curvature of the object side and 
image side lens surfaces of the positive meniscus lens component in the 
first lens group G1 which is most adjacent to the object side, d.sub.1 is 
the center thickness of said positive meniscus lens component, f is the 
focal length of the entire system, d.sub.F is the center thickness of the 
meniscus lens component in the third lens group G3 which has its convex 
surface facing the object side, d.sub.R is the center thickness of the 
negative lens component in the third lens group G3, and d.sub.A is the air 
space between said meniscus lens component and said negative lens 
component. 
Generally, in the conventional dry system plan objective lens having a flat 
image plane, a lens surface of strong negative refractive power having its 
concave surface facing the object is provided most adjacent to the object 
side. This lens surface of strong negative refractive power acts very 
effectively to render the Petzval sum negative and moreover, because of 
its being near the object, the amount of refraction of light rays in this 
surface is small and this surface hardly adversely affects aberrations for 
its strong refractive power. However, in order not to adversely affect 
aberrations, this lens surface need be very near the object surface and 
for this reason, the working distance has unavoidably been small. Usually, 
the lens which is most adjacent to the object side and which has such a 
lens surface having its concave surface of strong refractive power facing 
the object side has a very thick meniscus shape as compared with the 
radius of curvature thereof, the center thickness of this lens is 
substantially the same as the value of the radius of curvature of the 
image side lens surface of this lens having its convex surface facing the 
image side, and the radius of curvature of the concave surface of this 
lens has a value of 50-60% of that of the convex surface. The correlation 
between these values maintains a substantially constant rate irrespective 
of the magnification and numerical aperture of the objective lens and, in 
a dry system plan objective lens, it will be understood that such a 
construction of the lens which is most adjacent to the object side is 
requisite and has performed a great role. 
In the present invention, the construction of the conventional dry system 
plan objective lens as described above cannot be used to secure a long 
working distance and therefore, a positive meniscus lens having the 
construction as prescribed by conditions (1) and (2) above is disposed 
most adjacent to the object side and, for the correction of Petzval sum 
which will be undercorrected thereby, a lens group of negative refractive 
power having the construction as prescribed by condition (3) is provided 
as the third lens group. Accordingly, the present invention is 
characterized chiefly by the constructions of the positive meniscus lens 
in the first lens group which is most adjacent to the object side and the 
third lens group. 
Condition (1) above is for obtaining a long working distance and preventing 
extreme aggravation of Petzval sum. When the relation in magnitude between 
the absolute values of r.sub.1 and r.sub.2 shown in condition (1) is 
reversed, use is made of an aberration correcting technique similar to 
that used for the conventional objective lens described previously and 
therefore, it becomes basically impossible to maintain a long working 
distance. Also, if the absolute value of r.sub.2 becomes smaller than the 
focal length f of the entire system, it will be advantageous for the 
correction of spherical aberration and chromatic aberration, but Petzval 
sum will be excessively great in the positive sense and will be difficult 
to correct. 
If the left-hand sign of inequality of condition (2) is reversed, the 
radius of curvature r.sub.1 of the lens surface which is most adjacent to 
the object side will become relatively too great and will therefore 
aggravate spherical aberration and will also make the correction of 
Petzval sum difficult and thus, good correction will become difficult even 
by the rearward third lens group. If the right-hand sign of inequality of 
condition (2) is reversed, the radius of curvature of the lens surface 
which is most adjacent to the object side will become relatively small and 
will therefore impart a good influence to the correction of Petzval sum, 
but the spherical aberration created in this lens surface will become 
great and moreover, it will become difficult to maintain a great working 
distance. 
In subsequence to such a positive meniscus lens which is most adjacent to 
the object side, the first lens group for converting the light flux from 
the object into a convergent light flux need have two or three lens 
components and should desirably have at least one more positive meniscus 
lens component and one more biconvex positive lens component. It is also 
desirable to provide a cemented surface in at least the biconvex lens 
component in the first lens group to correct chromatic aberration. 
The second lens group basically has the function of correcting chromatic 
aberration and therefore, the refractive power thereof may be small as 
compared with the refractive powers of the other lens groups and, to 
provide an apochromat objective lens, it is desirable to constitute this 
lens group by a well-known triplet cemented lens component. 
The third lens group G3, as previously described, has a meniscus lens 
component having its convex surface facing the object side and a negative 
lens component disposed rearwardly thereof with an air space therebetween, 
and by condition (3) above, it causes the light flux passed through the 
second lens group to converge into a small light flux diameter and makes 
the image distance into a predetermined value and also corrects the image 
plane well with Petzval sum as negative. If the center thickness d.sub.F 
of the meniscus lens component in the third lens group G3 having its 
convex surface facing the object side becomes smaller, it is necessary to 
increase the air space d.sub.A between it and the subsequent negative lens 
component. To converge the light flux from the second lens group G2 by a 
predetermined amount, the value of the sum of the center thickness d.sub.F 
and the air space d.sub.A must be substantially constant and the greater 
is the value of this sum, the more intensely the light flux can be 
converged. If condition (3) is departed from, the convergence of the light 
flux will become insufficient and as a result, the image distance will 
become too long. In this case, it is possible to reduce each element of 
the lens system by a certain magnification to thereby reduce the full lens 
length including the image distance, but if this is done, the working 
distance will also be reduced by this magnification. 
Thus, by condition (3), in the third lens group G3, the light flux from the 
second lens group G2 is converged into a small light flux diameter, but it 
is desirable that the height of the paraxial ray from the on-axis object 
point emergent from the third lens group G3 be about 1/3, preferably, in 
the range of 1/4-1/2, of the height at which such ray enters the third 
lens group. 
The negative lens component rearwardly positioned in the third lens group 
G3 receives the light flux converged and stopped down by the meniscus lens 
component which is more adjacent to the object side than said negative 
lens component, and imparts a suitable magnification thereto and at the 
same time, contributes to good correction of the image plane with Petzval 
sum as negative. 
It is possible to cause a part of the action of this negative lens 
component to be borne by the image side surface of the forward meniscus 
lens component in the third lens group G3, namely, the diverging lens 
surface having its concave surface facing the image side, and this is 
particularly effective in a case where the center thickness of the 
meniscus lens component is great. It is desirable to provide a cemented 
surface having its convex surface facing the image side in the meniscus 
lens component in the third lens group G3 which is adjacent to the object 
side and accordingly, it is desirable to constitute this lens component by 
cementing a biconvex positive lens and a biconcave negative lens together. 
It is desirable that the negative lens component in the third lens group 
G3 have on the object side thereof a lens surface having its concave 
surface facing the object side, and it is also desirable to provide a 
cemented surface having its convex surface facing the object side. 
Where the microscope objective lens of the present invention having the 
basic construction as described above is used to effect a microscopic 
examination with a parallel flat plate such as a cover glass or a culture 
container disposed between an object O and the first lens group G1, the 
second lens group G2 is provided so as to be movable on the optical axis 
relative to the first lens group G1 and the third lens group G3, whereby 
the imaging performance deteriorated by a variation in the thickness of 
the parallel flat plate can be corrected. More specifically, where the 
thickness of the parallel flat plate P such as a cover glass or a culture 
container is greater than a predetermined design standard value, the 
second lens group G2 is moved toward the third lens group G3 and where the 
thickness of the parallel flat plate is smaller than the predetermined 
design standard value, the second lens group G2 is moved toward the first 
lens group G1, whereby an aberration corrected condition similar to that 
at the design standard value can always be maintained. To enable such 
aberration correction to be accomplished, it is of course necessary to 
provide in advance a space in which the second lens group G2 can be moved 
along the optical axis between the first lens group G1 and the third lens 
group G3. 
In order that the correction as described above may be well accomplished, 
the aberration structure of each lens group must be as follows. The first 
lens group G1 is endowed with a strong converging action and a 
considerably great negative spherical aberration. The second lens group G2 
is endowed with a positive spherical aberration which substantially 
offsets the negative spherical aberration created in the first lens group 
G1. The third lens group G3 of negative refractive power corrects the 
Petzval sum of the entire system and maintains the planarity of the image 
plane. It is desirable to provide in the third lens group G3 two lens 
surfaces opposed to each other with concave surfaces facing each other 
with an air space therebetween and correct Petzval sum by the diverging 
action in these two concave surfaces, but this is not essential when it is 
a principal object to correct spherical aberration. 
With such a basic structure as the standard, spherical aberration can be 
varied by the second lens group G2 being moved relatively on the axis 
between the first lens group G1 and the third lens group G3. That is, the 
second lens group G2 of relatively small refractive power is positioned in 
the convergent light flux emerging from the first lens group G1 and 
therefore, if the second lens group G2 is moved more toward the third lens 
group G3 than the reference position thereof, the height at which the 
convergent light flux cuts the second lens group G2 will become lower than 
that at the reference position and the amount of positive spherical 
aberration created in the second lens group G2 will decrease. Conversely, 
if the second lens group G2 is moved more toward the first lens group G1 
than the reference position thereof, the height at which the convergent 
light flux cuts the second lens group G2 will become higher than that at 
the reference position and the amount of positive spherical aberration 
created in the second lens group G2 will increase. Accordingly, spherical 
aberration greatly fluctuated by the thickness of the parallel flat P such 
as a cover glass disposed between the objective lens and the object 
surface is corrected by movement of the second lens group G2. That is, if 
the thickness of the parallel flat plate P becomes greater, positive 
spherical aberration is created and, to correct this, the second lens 
group G.sub.2 may be moved toward the third lens group G3 to decrease the 
amount of positive spherical aberration in the second lens group G2. On 
the other hand, if the thickness of the parallel flat plate P becomes 
smaller, negative spherical aberration is created and therefore, the 
second lens group G2 may be moved toward the first lens group G1 to 
increase the amount of positive spherical aberration in the second lens 
group G2. Such a situation of aberration correction can be known also from 
the tertiary aberration coefficient of spherical aberration regarding a 
fourth embodiment which will be described later. 
The lens construction of each lens group will now be described. As in the 
embodiment shown, the first lens group G1 has a considerably strong 
positive refractive power for converting the light flux from the object 
into a convergent light flux and for this purpose, it is desirable that 
the first lens group have at least three positive lens components. Of 
these lens components, the positive lens which is most adjacent to the 
object side need be of a meniscus shape having its concave surface facing 
the object side. As regards the second positive lens, it is desirable that 
the image side surface thereof be of sharper curvature, and it is 
desirable to provide a cemented surface in at least one of the three 
positive lenses. The second lens group G2 has a relatively weak refractive 
power and the function of creating spherical aberration greatly in the 
positive sense and therefore, as in the embodiment shown, it is formed by 
cementing together a negative meniscus lens convex toward the object side, 
a biconvex positive lens and a negative lens. The shape of the second lens 
group as a whole is like that of a positive lens, but the refractive index 
of the negative meniscus lens is higher than that of the biconvex lens and 
therefore, the second lens group as a whole has a weak negative refractive 
power. This is because the second lens group G2 is intended to have the 
function of correcting spherical aberration in the positive sense. 
In each embodiment which will be described later, the second lens group G2 
is constructed as a lens group of weak negative refractive power 
comprising three lenses cemented together, but it is also possible to 
endow the second lens group with a weak positive refractive power. As a 
further alternative, the second lens group may be divided into and 
comprised of a plurality of groups such as positive and negative lens 
groups. 
The third lens group G3 directed chiefly to the correction of Petzval sum 
has a negative refractive power as a whole, but it is desirable that the 
forward group G31 of the third lens group have a weak positive refractive 
power and the rearward group have a weak negative refractive power. The 
concave surface Ra of the forward group G31 which is most adjacent to the 
image side and the concave surface Rb of the rearward group which is most 
adjacent to the object side function as the opposed concave surfaces in 
the third lens group as described previously. It is desirable that the 
forward group G31 of the third lens group be formed by cementing together 
a positive lens, a negative lens and a positive meniscus lens having its 
convex surface facing the object side. However, the negative lens and the 
positive meniscus lens cemented together in the forward group G31 form a 
hyperchromatic lens and therefore, the direction of this cemented surface 
may also be reversed. Also, it is desirable that the rearward group G32 of 
the third lens group be formed by cementing together a thick biconcave 
negative lens and a biconvex positive lens. 
Some embodiments of the microscope objective lens according to the present 
invention will hereinafter be described in detail. First, second and third 
embodiments of the present invention are for use in a reflection 
illumination type microscope such as a metal microscope, and a fourth 
embodiment is for use in a transmission illumination type microscope such 
as a biological microscope. In the fourth embodiment, the second lens 
group is provided movably along the optical axis in accordance with a 
variation in the thickness of the cover glass. 
FIG. 1 shows the lens construction of the first embodiment of the present 
invention. In FIG. 1, to facilitate the understanding of the function of 
each lens group, marginal light rays from the on-axis object point are 
shown. This first embodiment has a high magnification and a high numerical 
aperture (N.A.), i.e., a magnification of 60 and N.A. of 0.7, and yet the 
distance d.sub.0 between the object surface and the vertex of the foremost 
lens surface is 1.77 times as great as the focal length of the objective 
lens, and this embodiment has a working distance as great as about 5 mm in 
practical use. 
The second embodiment shown in FIG. 2 is of a construction in which a 
positive lens is added to the first lens group in the first embodiment. 
This embodiment also has a magnification of 60 and N.A. of 0.7, but the 
distance d.sub.0 between the object surface and the vertex of the foremost 
lens surface is more than 1.8 times the focal length of the objective lens 
and in practice, this embodiment has a working distance as long as about 5 
mm. 
The third embodiment of FIG. 3 is a high magnification objective lens 
having a magnification of 100 and N.A. of 0.9 and, as compared with the 
construction of the second embodiment of FIG. 2, this embodiment is 
characterized in that the second meniscus lens component in the first lens 
group G1 is a cemented lens component having a center thickness greater 
than that of the first meniscus lens component and that the spacing 
between the meniscus lens component in the third lens group G3 and the 
subsequent negative lens component is smaller. The distance d.sub.0 
between the object surface and the foremost lens surface in this objective 
lens is 75% of the focal length and in practice, this embodiment has a 
very great working distance of about 1 mm as an objective lens of this 
magnification. 
The numerical data of the above-described embodiments will be shown below. 
In the tables below, the left-hand numbers represent the order from the 
object side, r represents the radius of curvature of each lens surface, d 
represents the center thickness and air space of each lens, nd represents 
the refractive index of each lens for d-line (.lambda.=587.6 nm), and .nu. 
represents the Abbe number of each lens. Also d.sub.0 represents the 
distance between the object surface and the vertex of the foremost lens 
surface. 
TABLE 1 
______________________________________ 
(First Embodiment) 
Focal length f = 1.0 N.A. = 0.7 
Magnification 60 d.sub.0 = 1.77360 
No. r d nd .nu. 
______________________________________ 
1 -3.2947 0.8020 1.62254 
53.07 
2 -1.8908 0.0086 
3 -32.1403 0.7877 1.49805 
82.32 G1 
4 -3.7418 0.0086 
5 6.0714 0.5442 1.75716 
31.71 
6 3.3514 1.8619 1.43388 
95.57 
7 -4.8696 0.0286 
8 6.5873 0.2864 1.75031 
35.26 
9 2.9229 1.7187 1.43388 
95.57 G2 
10 -3.4367 0.2864 1.61705 
54.13 
11 -18.1066 0.0086 
12 2.3409 1.7616 1.49805 
82.32 
13 -19.1940 0.3724 1.71763 
48.03 
14 3.7356 2.7785 G3 
15 -1.2321 0.7161 1.74868 
52.36 
16 3.3732 0.4297 1.74127 
27.66 
17 -3.3732 
______________________________________ 
TABLE 2 
______________________________________ 
(Second Embodiment) 
Focal length f = 1.0 N.A. = 0.7 
Magnification 60 d.sub.0 = 1.84572 
No. r d nd .nu. 
______________________________________ 
1 -3.5289 0.7072 1.52359 
65.12 
2 -1.9368 0.0144 
3 -20.3731 0.6929 1.49782 
82.28 
4 -3.7730 0.0144 
5 28.1836 0.6374 1.49782 
82.28 G1 
6 -6.9326 0.0135 
7 12.4790 0.2591 1.74950 
35.19 
8 3.9457 1.5581 1.43388 
95.57 
9 -5.0967 0.0086 
10 6.2046 0.1710 1.74590 
43.12 
11 3.2435 1.6908 1.43388 
95.57 G2 
12 -4.2575 0.2036 1.74443 
49.46 
13 -11.5554 0.0146 
14 2.4378 2.6760 1.49782 
82.28 
15 -5.3823 0.3221 1.61266 
44.40 
16 1.3344 2.2086 G3 
17 -1.5760 0.3360 1.71300 
53.97 
18 4.4631 0.4336 1.78934 
26.26 
19 -3.2816 
______________________________________ 
TABLE 3 
______________________________________ 
(Third Embodiment) 
Focal length f = 1.0 N.A. = 0.9 
No. r d nd .nu. 
______________________________________ 
1 -3.0820 0.9782 1.74868 
52.36 
2 -1.6190 0.0245 
3 -3.5948 0.9196 1.75471 
27.66 
4 21.5169 2.2108 1.49805 
82.32 
5 -3.0604 0.0489 G1 
6 185.0551 2.1032 1.61420 
30.73 
7 -7.1415 0.0245 
8 10.6872 1.7951 1.75710 
31.70 
9 4.5979 2.8858 1.43388 
95.57 
10 -8.6080 0.0245 
11 15.1363 0.9782 1.75031 
35.26 
12 4.2152 2.9347 1.43388 
95.57 G2 
13 -4.1580 0.3913 1.48743 
70.24 
14 -20.4677 0.8755 
15 3.4756 4.4021 1.49805 
82.32 
16 -10.7367 1.2717 1.71356 
53.98 
17 1.8403 0.5136 G3 
18 -2.0436 0.4158 1.69758 
55.72 
19 2.0436 0.6848 1.61400 
30.72 
20 -3.0819 
______________________________________ 
The various aberrations when the objective lenses of the first, second and 
third embodiments are used at their respective magnifications are shown in 
FIGS. 4, 5 and 6, respectively. These aberration graphs are the 
performance evaluations when, with respect to the objective lens of each 
embodiment, the full length from the object surface to the image plane is 
245 mm. In these aberration graphs, the spherical aberration, astigmatism, 
coma and distortion for d-line (.lambda.=587.6 nm) are shown and, in the 
graph of spherical aberration, the spherical aberrations for C-line 
(.lambda.=656.3 nm) and F-line (.lambda.=486.1 nm) are also shown. 
From these aberration graphs, it is apparent that in each exbodiment, in 
spite of its having a great N.A. and a great working distance, the 
planarity of the image plane is good and any of the various aberrations is 
corrected very well. 
FIG. 7 shows the lens construction of a fourth exbodiment of the present 
invention. This embodiment is an example in which the second lens group G2 
may be moved relative to the first and third lens groups G1 and G3 in 
accordance with a variation in the thickness of the cover glass to thereby 
correct any fluctuation of aberrations resulting from the variation in the 
thickness of the cover glass. The numerical data of the fourth embodiment 
are shown in Table 4 below. In the Table 4, the left-hand numbers 
represent the order from the object side, and d.sub.0 represents the 
distance from the vertex of the foremost lens surface of the objective 
lens to the surface of the cover glass. 
TABLE 4 
__________________________________________________________________________ 
(Fourth Embodiment) 
Focal length f = 1.0 N.A. = 0.7 
Magnification .beta. = 61 
Center thickness 
Radius of 
and air space of 
Refractive 
Abbe 
No. 
curvature r 
each lens d 
index n 
number .nu. 
__________________________________________________________________________ 
.infin. 
(0.4548) 1.52216 
58.8 Cover 
.infin. 
(d.sub.0 = 1.3859) 
1.0 glass P 
1 -2.8310 
0.9475 1.713 54.0 L.sub.1 
2 -1.9995 
0.0758 1.0 
3 +24.7522 
0.8527 1.49782 
82.3 L.sub.2 
G1 
4 -4.5072 
0.0379 1.0 
5 +5.6203 
0.5685 1.75692 
31.7 L.sub.3 
6 +3.6523 
2.0465 1.43388 
95.6 
7 -6.4804 
(d.sub.7 = variable) 
1.0 
8 +6.5565 
0.2842 1.6968 
55.6 L.sub.4 
9 +2.8716 
1.8191 1.43388 
95.6 L.sub.5 
G2 
10 -2.8716 
0.3411 1.62041 
60.3 L.sub.6 
11 -15.9184 
(d.sub. 11 = variable) 
1.0 
12 +2.5017 
0.9664 1.49782 
82.3 L.sub.7 
13 -142.5109 
0.2274 1.71736 
29.5 L.sub.8 
G31 
14 +1.5694 
0.9475 1.713 54.0 L.sub.9 
15 +3.0702 
2.9750 1.0 G3 
16 -1.4057 
1.0820 1.713 54.0 L.sub.10 
G32 
17 +1.7331 
0.7580 1.72825 
28.3 L.sub.11 
18 -3.8671 1.0 
__________________________________________________________________________ 
Thickness of cover 
glass d.sub.0 
d.sub.7 
d.sub.11 
__________________________________________________________________________ 
0.2653f 1.5130 1.2128 
1.7812 
0.4548f 1.3859 1.8570 
1.1370 
0.6443f 1.2584 2.6529 
0.3411 
__________________________________________________________________________ 
FIGS. 8A, 8B and 8C show the various aberrations when the objective lens of 
the fourth embodiment has been proportionally enlarged so that as a 
practical objective lens of a magnification of 61, f=2.64 mm, that is, the 
distance from the object surface to the image plane is 195 mm, FIG. 8A 
showing the various aberrations in a state in which the thickness of the 
cover glass is relatively small, say, 0.7 mm (0.2653f), FIG. 8B showing 
the various aberrations in the standard in which the thickness of the 
cover glass is 1.2 mm (0.4548f), and FIG. 8C showing the various 
aberrations in a state in which the thickness of the cover glass is as 
great as 1.7 mm (0.6443f). In these aberration graphs, spherical 
aberration, astigmatism, distortion and coma are shown and the standard 
light ray is d-line (.lambda.=587.6 nm), and C-line (.lambda.=656.3 nm) 
and F-line (.lambda.=486.1 nm) are also shown to show chromatic spherical 
aberration. In these Figures, the value of y represents the image height. 
From these aberration graphs, it is clear that the objective lens of the 
fourth embodiment has a great working distance and a great numerical 
aperture of N.A. =0.7 and yet always maintains an excellent imaging 
performance over a wide range of the thickness of the cover glass, i.e., 
0.7 mm to 1.7 mm. 
The tertiary aberration coefficients of spherical aberration in the fourth 
embodiment are shown in Table 5 below. In Table 5, the coefficients in the 
three cases of the thickness of the cover glass shown in Table 4, and the 
left-hand numbers represent the order of the lens surfaces from the object 
side. According to Table 5, as the cover glass is thicker, the tertiary 
aberration coefficient of spherical aberration in the surface of the cover 
glass is of a greater value in the negative sense, and this endorses that 
the spherical aberration of the cover glass increases in the positive 
sense. It is apparent that even if the cover glass becomes thicker, the 
tertiary aberration coefficients of spherical aberration in the first G1 
and the third lens group G3 hardly vary, whereas the tertiary aberration 
coefficient of spherical aberration in the second lens group G2 varies 
greatly and the second lens group G2 substantially offsets the variation 
in the aberration coefficient in the surface of the cover glass. As a 
result, the tertiary aberration coefficient of spherical aberration in the 
entire system is a substantially constant small value even if the 
thickness of the cover glass is varied. This endorses that spherical 
aberration is always corrected well, and is well coincident with the 
spherical aberration graphs shown in FIGS. 8A, 8B and 8C. 
TABLE 5 
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Tertiary Aberration Coefficients of Spherical 
Aberration 
______________________________________ 
Thickness of cover 
0.2653f 0.4548f 0.6443f 
glass 
Surface of cover 
-0.09260 -0.15913 -0.22604 
glass 
1 0.13241 0.13320 0.13409 
2 -0.06343 -0.06387 -0.06435 
3 0.31510 0.31600 0.31695 
4 0.01821 0.01786 0.01744 
5 0.29924 0.30044 0.30177 
6 -0.27883 -0.27957 -0.28036 
7 0.14135 0.14064 0.13977 
1st group G1, total 
(0.56405) (0.5647) (0.56531) 
8 0.00097 0.00045 -0.00003 
9 -0.16103 -0.13800 -0.11330 
10 -0.30704 -0.26882 -0.22710 
11 0.01782 0.01629 0.1455 
2nd group G2, total 
(-0.44928) (-0.39008) (-0.32588) 
12 0.11800 0.11725 0.11648 
13 -0.03040 -0.03020 -0.03001 
14 -0.00174 -0.00173 -0.00172 
15 0.00230 0.00228 0.00227 
16 -0.10465 -0.10398 -0.10330 
17 0.00279 0.00277 0.00275 
18 0.00381 0.00378 0.00376 
3rd group G3, total 
(-0.00989) (-0.00983) (-0.00977) 
Sum total of entire 
0.01228 0.00565 0.00362 
system 
______________________________________ 
The basic characteristic of the aberration fluctuation correction by the 
parallel flat plate as in the above-described fourth embodiment lies in 
that, as already described, the light flux from the object is converted 
into a convergent light flux by the first lens group and the converged 
state of the light flux is substantially maintained even after the light 
flux passes through the second lens group. The light flux is condensed at 
a predetermined image plane position after it passes through the third 
lens group. Therefore, as shown in FIG. 9, the height at which the 
paraxial ray from the on-axis object point cuts each lens group is highest 
in the first lens group G1 and lower in the order of the second G2 and the 
third lens group G3. When the maximum values of the heights at which the 
paraxial ray from the on-axis object point cuts the first and second lens 
groups are h.sub.1 and h.sub.2 and the height at which the paraxial ray 
emerges from the third lens group is h.sub.3, it is necessary that h.sub.1 
&gt;h.sub.2 &gt;h.sub.3. Accordingly, as regards the effective diameters of the 
respective lens groups, the first lens group is greatest and the third 
lens group is smallest, and it is desirable to construct the lens groups 
to such a degree that 6h.sub.3 &gt;h.sub.1 &gt;2h.sub.3. Where the present 
invention is applied to an objective lens of higher magnification, the 
value of the ratio of h.sub.1 to h.sub.3 becomes greater, and where the 
present invention is applied to an objective lens of lower magnification, 
the value of the ratio of h.sub.1 to h.sub.3 becomes smaller. Also, it is 
desirable that the gradient of the light ray passed through the first lens 
group G1 have a value five to ten times as great as the gradient of the 
light ray passed through the entire system. This is because, as previously 
described, when the second lens group G2 is moved along the optical axis 
between the first lens group G1 and the third lens group G3, the amount of 
correction of spherical aberration differs depending on the height of the 
light ray entering the second lens group G2, and this gradient has the 
tendency of becoming greater for an objective lens of higher magnification 
and becoming smaller for an objective lens of lower magnification. Also, 
the construction in which the effective diameter of the first lens group 
is greatest is advantageous for making the working distance great. 
According to the present invention, as described above, there is acheived a 
dry system objective lens of medium or high magnification having a great 
working distance and yet having an excellent imaging performance. For 
example, as an objective lens of 60 times, the objective lens of the 
present invention has a working distance of about 5 mm, and as an 
objective lens of 100 times, the objective lens of the present invention 
has a working distance of about 1 mm, and thus, the objective lens of the 
present invention can have a working distance about ten times as great as 
that of the conventional objective lens. Accordingly, the undesirable 
possibility that during microscopic examination, the fore end of the 
objective lens touches the object surface to injure the latter is reduced, 
and the operability is also improved very much. 
Further, the object lens according to the present invention is an objective 
lens of high magnification having a great numerical aperture and yet 
suffering from less deterioration of aberration even if the thickness of 
the parallel flat plate such as the cover glass is greatly varied, an can 
always maintain a good imaging performance. Also, the refractive power of 
the lens group moved for aberration correction is weak and therefore, the 
variations in the principal point and focus position of the objective lens 
are slight and thus, even if the correcting ring is operated, the amount 
of out-of-focus is slight, and this is convenient.