Year: 1899

page_text: Date of Application, 31st Aug., 1899-Accepted, 7th Oct., 1899
I, HAROLD DENNIS TAYLOR, of Trenfield, Holgate, York, Optician, and a Director
of the Firm of T Cooke & Sons Limited, Buckingham Works, York, do hereby declare
the nature of this invention and in what manner the same is to be performed, to
be particularly described and ascertained in and by the following statement :-
My invention consists of an improved form of achromatic eye piece of the negative
lens or Galilean form
It is a well known objection to the Galilean form of telescope such as is employed
for binocular, field, or opera glasses that the apparent field of view is generally a very
nairow one unless the focal lengths of the object glasses are made excessively short
compared with their aperture. Even then the apparent field of view is not nearly so
large as that which can be obtained by the use of a good Huygenian eye piece; but
the use of the latter for viewing terrestrial objects is precluded, owing to its neces-
sarily showing objects upside down, unless reflectors or prisms are first introduced for
erecting the image. My form of eye piece not only gives an apparent field of view
about 1 times what is obtained by a single negative lens of equal power when used
with the same objective, but also can be made to vary in its magnifying powers by
simply slightly altering the distance between the two simple lenses of which it is
composed. In this way it can be arranged that by turning round the cell carrying
the eye lens upon a screw of suitable pitch, less than one complete turn will suffice
to alter the magnifying power from 5 up to 10 diameters in the case of higher power
eye pieces, or, say, from 3 to 5 diameters in the case of lower powers. When set to
the highest power the apparent field is 1 and one-third to 1 times what is obtained
by a simple negative lens of equal power used on the same objective, and when set to
the lowest power the apparent field of view 18 about double what is obtained by a
simple negative lens of equal power and on the same objective. Thus, when used
with ordinary field glass objectives I can obtain an apparent field of view as large as
that given by the usual forms of Huygenian eye pieces or four lens erecting eye
pieces. The achromatism of my eye piece for the edges of its field depends upon
the condition underlying the Huygenian eye piece, namely that the two lenses
must be placed at a distance apart equal to one half the sum of their focal lengths,
if both are positive lenses, or half the difference of their focal lengths if one is a
positive lens and the other a negative lens. It also presents the further advantage
of coming considerably closer to the objective when in focus than a single negative
eye lens of equal power, especially when set for the lower powers. Thus the
telescope is made shorter and more portable. Experiment and calculation have
led me to the conclusion that it is essential to the best results obtainable from
my invention that it should consist of a meniscus field lens of a crown glass of low
refractive & dispersive powers closely followed by a double concave eye lens of about
18 times the power of the field lens and made of a dense flint glass of high
refiactive & dispersive powers
In the accompanying drawing Fig. 1 is a longitudinal section through my eye
piece, showing the meniscus field lens F mounted in the fore part A of the eye piece
and the double concave eye lens E mounted in the after part B of the eye piece.
The fore part A screws into the draw tube of the held glass in place of the usual
eye piece socket. A is screwed at 8 and the after part B screws easily but firmly

page_text: over this. Thus by screwing round the after part B the eye lens can be made to
approach or recede from the field lens F, and the power of the eye piece be varied
thereby I will now give the curves and other particulars of one of these -eye
pieces, which is of just 2 inches equivalent focal length, when the two lenses are
at such a distance apart as to forin an aplanatic combination, that is free from
spherical aberration for an axial pencil of rays when used on an ordinary opera
glass objective of about 6 inches focal length.
The radi of curvature are given with a plus (+) sign for a convex surface and
with a minus sign (-) for a concave surface.
The meniscus field lens F is of a focal length equal to 18 times the focal length
of the eye-lens, and made of a boro-silicate crown glass having a refractive index
151 and of dispersive power
for the D ray
13
Its curves are to +245 in. and
The bi-concave eye lens
refractive index for the D ray
E
18
-
HI
for rays C to F.
63.5
29 in. radii finished diameter -35 in.
made of ordinary deuse flint glass having
162 dispersive power
1
B
for rave C to F. Its
36
curves are to 1.68 in. and ⚫21 in. radu Finished diameter
-
-
=
*25 inch.
The deep side of the eye lens (next eye) 18 ground down on a flat until the clear
aperture of the polished surface ineasures in. diameter The diaphragm D is
perforated with a sharp edged eye hole of between 08" and 10" clear diameter and
not more. It is important that this eye hole should lie as close as possible to the
eye lens E. It is unscrewed from the after part of eye piece, when it is required to
clean the outer surface of the eye lens.
From the above data the radi of curvatures for eye-pieces of any other focal
lengths may be deduced.
The eye piece is supposed to be tested upon the opera or field glass objective that
it is intended for and the distance between the two lenses adjusted until the eye piece
18 judged free from spherical aberration along its axis by the usual well known tests;
then its equivalent focal length will be just about 2 inches and it will magnify three
times if used with an objective of 6 inches focal length. But its distance inside the
main focus when set for clear vision for normal eyesight will be not 2 inches but 2.7
to 3 inches, and thus the telescope 18 shorter by upwards of an inch than it would
otherwise be.
If the two lenses are more separated and the magnifying power decreased, then
the eyepiece will have to be advanced still further within the main focus, when in
focus.
If it is required to make the eye-piece of variable power, then the screw S must
not be made too fine in pitch, so that less than one complete turn of the after part B
will suffice to bring about all the change of power required, and so that the after
part B may rotate backwards and forwards between two stops, or rather the two sides
of one stud like that shown at V may act as a stop against which a tail T left pro-
jecting from the lower edge of the after part B may be brought up.
For an eye
piece of 2 in. focal length the pitch of the screw should be about 15 threads per inch.
Graduations should be engraved on the cylindrical surface of B and a reading mark
be engraved on V. These graduations should preferably indicate the magnifying
powers which the eye-pieces yield when set to the particular marks and when used
with their proper objectives. These magnifying powers are obtained by dividing
the equivalent focal length of the eye-piece when set to any particular mark into the
focal length of the objective it is intended to use it with; and in order to measure
the equivalent focal length of these eye pieces, a slightly modified Cavallo's dyna-
mometer 18 almost indipensable. A positive eye piece of the Ramsden or Hay genian
type will form behind it a real aerial image of any object place in front of it, and its
magnifying power and consequent focal length are generally obtained by measuring
the size of the aerial image of the objective which it forms. But a negative eye lens
or eye piece of the Galilean form or the form described herein, forms only a virtual

page_text: image I (Fig. 1) of the objective on other object placed in front of it, and in order to
measure this virtual image a compound dynamometer microscope of long focus must
be used Supposing O is the objective of this microscope having its conjugate foci
at I and R, at which latter place a properly graduated scale is placed, then an image
(more or less enlarged or reduced in a known degree) of the image I will be formed
at R on the scale, by reference to which latter its diameter can be accurately
measured, and so the equivalent focal length of the eye piece obtained
The method I find most convenient is to put in the principal focus of a long
collimator a white disc of a diameter equal to exactly Toth part of the equivalent
focal length of the collimator objective. This disc faces the latter and is illuminated,
while the eye piece whose focal length is required is placed in front of and facing
the collimator; then it forms a vittual umage I of the white dise whose diameter
can be accurately measured by means of the compound dynamometer Then it is
obvious that the equivalent focal length of the eye piece will be 100 times the
diameter of the virtual image I of the disc.
When used for binocular telescopes it is of importance to have the eye pieces
accurately paired as to focal length and to be accurately graduated, if of variable
powers, and this can be easily and expeditiously carried out by means of the
above described methods, whereby the focal lengths of the eye pieces may be
adjusted to give the required magnifying powers In some cases it may be
preferable to have the eye pieces of one fixed power, in which case no rotation
of the after part carrying the eye lens 1s to be allowed except for the purpose of
first adjustment.
The metal mount, Bockets for the lenses, and the screwed surfaces, must be
carried out with exactitude so as to ensure truth of alignment between the lenses,
more especially in the case of the higher powers, and the lenses themselves must be
accurately centred and edged.
I have above given full particulars of the best form of my eye piece which I have
hitherto worked out, but it is capable of considerable variations in the curves and
the materials consistently with passably good results being obtained. The field
lens may be made of ordinary crown glass or plate glass or even of a glass of high
refractive power with or without a high dispersive power, and the eye lens may be
made of a glass of high refractive index and low dispersive power like Dense Barium
Crown, in which case however the achromatisin at the edge of the field will not be
so perfect; or the eye lens may be made of a crown glass of low refractive power
and correspondingly deeper curves, in which case a greater separation between the
two lenses will be required in order to make the combination free from spherical
aberration. This will lead to a loss of magnifying power and more imperfect
achromatism but greater apparent field. But I have adopted the curves and
materials herein described as the best combination known to me for satisfying the
conditions of achromatism, aplanatism, flatness of field and freedom from coma and
distortion consistently with the largest possible apparent field of view being attained,
and without having to resort to impracticably deep curves for the eye lens.
Having now particularly described and ascertained the nature of my said in-
vention and the manner in which the same is to be performed, I declare that what
I claim 18:
(1) As a low power negative eye piece for binocular or other telescopes a com-
bination for the purposes herein described consisting of a positive meniscus or
approximately convexo plane field lens closely followed by a double concave or
approximately plano concave eye lens of a power considerably greater than that of the
field lens, forming a substantially aplauatic and large field eye piece, whose
magnifying power may be varied, if required, by simply altering the distance between
the two lenses, by rotating the after part of the eye piece, which carries the eye lens,
upon a screw of suitable pitch.
(2) As a low power negative eye piece for binocular or other telescopes, a com-

page_text: bination substantially as and for the purposes herein described consisting of a positive
meniscus field lens inade of a crown glass of low refractive and dispersive powers
closely followed by a double concave eye lens of a power approximately 1-8 times that
of the field lens, and made of ordinary dense flint glass, forming a substantially
aplanatic and large field eye-piece whose magnifying power may be varied, if required,
by simply altering the distance between the two lenses, by rotating the after part of
the eye-piece, which carries the eye lens, upon a screw of suitable pitch.
1000
H. DENNIS TAYLOR.