Year: 1897

page_text: We, THE SOCIÉTÉ, R. & C. Avizard, of 21, Boulevard Poissonniere, Paris, in the
Republic of France, Opticians, 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 :-
.1
Before stating the nature of the invention forming the subject of the present
application we deem it our duty to mention that it is the outcome of the enquiries
and labours of Captain Daubrasse.
In order to enable the nature of the invention to be clearly understood, we will
in the following statement recapitulate in the first part some of the principles
of optics which bear on the question here involved, and remind the reader in the
second part how the ordinary terrestial ocular or eye-piece is constructed and we will
lastly describe the invention, turning our attention more especially to those essential
features which distinguish it from what was previously known.
The ocular or eye-piece of a telescope is a combination of lenses, or "compound lens,"
designed to give the virtual image, as perceived by the observer's eye, of the real
image furnished by the object-piece, or "objective."
eye piece scular,
110
matter of what description, should fulfit certain
conditions, upon which the following properties of the lenses are dependent, viz.
y proce
conditions, upon which the following properties of the lenses are dependent, viz.
·
1. THE IMAGE SHOULD BE CLEAR, OR APLANATIC, 'AXIALLY.
Taking a pencil of rays falling from one and the same point of the object viewed,
this pencil of rays, on being refracted by the object-piece, and then by the lenses of
the eye-piece, should, as it leaves the instrument, form a sufficiently aplanatic pencil,
that is to say, all the rays should virtually pass through the same point; compliance
with which condition will ensure a clear and sharp image:
1. THE IMAGE SHOULD BE CLEAR, OR APLANATIC, 'AXIALLY.
Taking a pencil of rays falling from one and the same point of the object viewed,
this pencil of rays, on being refracted by the object-piece, and then by the lenses of
the eye-piece, should, as it leaves the instrument, form a sufficiently aplanatic pencil,
that is to say, all the rays should virtually pass through the same point; compliance
with which condition will ensure a clear and sharp image:

page_text: II. THE OLD TERRESTRIAL OCULARS OR EYE-PIECES.
What is known as a terrestrial ocular or eye-piece is an ocular consisting of con-
vergent lenses wherein the inverted real image, given by the object-piece, produces
an erect virtual image.
In order satisfactorily to carry out the several conditions hereinabove named,
including that of turning images into the erect position, it is necessary to make up
the eye-piece of a number of lenses, so as to have a sufficient number of variations
in working our problems by calculation. Hitherto terrestrial eye-pieces have con-
sisted of four lenses, the forms and optical effects of which are as follows (see
Figure 1 of the accompanying drawings)
The inverted image A¹ B¹, given by the objective, of an object placed outside,
forms between the first lens 01 and its front focus F¹, and this lens (a plano-convex
lens presenting its plane surface to the incident. light) substitutes for it a virtual
image a b¹, and in the same direction as A¹ B¹. The rays sent forth from a b¹ which
next pass through the lens 02 (a plano-convex lens with its surfaces turned in the
same directions as those of the preceding lens),-a b¹ being at a greater distance
from 02, than its focal length ()2 F2-will produce a real image a² 62 pointing in the
opposite direction to that of the virtual image a' b¹ and, consequently, opposite also
to that of A¹ B¹.
The "system" or set formed by the two first leuses 01 02 of the ocular or eye-piece
therefore substitutes for the inverted and real image A¹ B₁ furnished by the objective,
another image a² 62 which is also real, but erect. This combination of the two
lenses O¹ and 02 has been given the name vehicle, while the lens 0%, which turns
the image back to the erect position, is described as the rectifying glass.
The two lenses 03 and 04 are plano-convex lenses, like those mentioned before, the
only difference being that they present their convex side to the incident light. The
lens 03 placed within the path of the rays, which otherwise would.: form
the image a² ², prevents this image from forming, substituting for it a real
image ab, directed in the same way as a² b². This lens 03 has been called the
field glass. And as the image a³ 63 forming between the lens 0 (the eyeglass)
and its forward focus, this last mentioned glass substitutes for it a virtual
image ab turned in the same direction and perceived by the eye placed
behind 04:
..
"
"
Figure 2 of the accompanying drawings shows both the position of the several
images and the course taken by a pencil of light emitted from a point A of the
object. The axis of the pencil undergoes a succession of refractions and meets the
main common axis of the set, successively, at the points K! K2 K3 K4, which are
nothing but the images of the optical centre of the objective in the "systems" con-
stituted, firstly, by 0¹ alone, then by 01 02 and then 01 02 03, and lastly 01 02 03 04,
that is to say, by the complete set forming the eye-piece.
I
If the centre of the pupil be placed at K, all the rays emerging from the instru
ment may enter the eye at the same time. This point K is therefore called the
ocular point, and an eye-mark conveniently placed at the front end of the
ocular marks the position which the eye should occupy in order to fulfil this
condition.
It will be understood that the many conditions hereinabove enunciated cannot
'be
fulfilled by the vehicle and by the combination of the field glass and eye-glass,
separately, and that they can only conceivably be carried out by the simultaneous
action of the complete system. The calculus whereby terrestrial oculars are deter-
mined admits of extensive development into which it is neither necessary nor
desirable to enter here. Suffice it to say that it allows for a certain latitude of which
advantage is taken in order to render the focus F2 of the lens 02 variable. This
latitude, however, is very limited in reality and as a matter of fact the variation.,of
that focus admitted in practice is only as between the value F1 of the focus 01 and a

page_text: the ocuIRT
increases with the value adopted for the relation
F9
F4
All the single lens oculars come very near to the proportions above given. If,
moreover, we take one of these two types (or any one of those which might be found
by calculation for an intermediate value then, obviously, the ocular will be the
F2
Fi
more powerful in proportion as F (and consequently the remaining focals) diminish.
The length L, being at a constant ratio to F1 for any given type an ocular of such
type will consequently be the shorter in proportion as it is more powerful.
As regards clearness of image, we have already seen that the spherical aberrations
of the lens 02 should be as far as practicable reduced.
In view of this, in some oculars the lens in question, instead of being given a
plano-convex shape, has received a bi-convex shape, whereby, given the position of the
image ab², the said aberrations were reduced.
Another solution of the problem consists in separately achromatising this lens, so as
still further to reduce its aberrations. In oculars where 02 is achromatic, the
eye-glass is also rendered achromatic. We here give the result of the calculation
made with a view to carrying out this combination, which, however, only applies to
F2

page_text: 111. THE NOVEL
TERRESTRIAL OCULAR.
Presuming as a matter of fact that in trying to simultaneously comply with all
the requirements enumerated under I, the number of "variables" at disposal plays an
important part, we have endeavoured to find whether, by adopting five convergent
lenses instead of four, for constructing the terrestrial eye piece, it is not possible:
1. To effect the corrections in a more extensive field.
2. To produce, by means of lenses, a strictly satisfactory pancratic eye piece
permitting of the magnifying power thereof being varied within more extensive
limits than has heretofore been the case, by displacing the lenses relatively to one
another according to a convenient law.
This research has led us, to combine an eye piece, composed of five convergent
lenses, of which Figure 3 shows the arrangement as well as the optic function of each
element of this eye piece.
The formation of the images is analogous to that of the eye piece with four
glasses a ba² b² and a³ 63. This latter is formed between 0 and its front focus and
gives the virtual image at bt, which is likewise to be formned between 0 and its
front focus, so as to give a virtual image a5 b6, perceived by the eye. In a word the
two latter glasses 0 and 05 form a combination which, from the point of view of
nature and of the position of the images, is analogous to what is termed in
astronomy "the positive eye piece," with this difference, however, that the curvatures
and the focals have no relation whatever with those of an astronomic eye piece,
which has to produce the aplanetism and the achromatism and the centre of
emission of which (optic centre of the objective) is real and placed forward, whereas
in our eye piece the centre of emission K is virtual and placed rearward.
The results of this combination are as follows:
1. A clear, convenient and considerably increased field.
2. The clearness, the aplanetism and achromatism are better in spite of the large
extent of the field,

page_text: 3. The pancratic eye piece can be produced under excellent conditions with the
same lenses for considerable differences of magnification (of simple, double or more)
on condition that at least two of the distances can be varied according to a convenient
law. The most favourable are d¹ and d².
4. In this eye piece with constant glasses, the shorter the eye piece the less
powerful it is, a result which is diametrically opposite that obtained by the eye piece
with four glasses.
20:0
**
We have calculated the elements of this eye piece in the two cases corresponding
to those; which are known with regard to the eye piece having four lenses, namely:
1. The lenses are all five simple lenses.
2. The lenses are': 0¹ 03 04
02 and 05
simple lenses.
achromatic lenses.
We will hereinafter quote the elements of each of these eye pieces:
1. Eye piece with five glasses, all of which are simple...
3
With regard to suitably chosen focals we have found that if the lenses be all
simple, it is necessary for producing the pancratic eye piece to have three dis:
tances d¹ d² and d³ varied. We have on our part adopted the relation=
because in the eye piece with five glasses as well as in that with four glasses, the
length of the eye piece, is, under otherwise equal conditions, smaller than in the case
of higher values of this relation.
↑
F1.
The focals of the 'different lengths as well as the distances separating them, aré
given by the following table : !.
24.1*

page_text: be achromatized separately, the following table will indicate the focals and the
distances of the different lenses:
2
3
4
Fi
1
K2
1
} & =
= 1
1.14
1.28
1.42
F3
d²
2.08
1-8
1.53
1.25
F¹
=
0.94 d³
=
0.64
0.64
0.64
0.64
F5 0.61 d = 0.28
0.28
0.28
0-28
2
1.7
1.5.
1:2
In this eye piece, the retainable value of the enlarged field may reach about 1·35.
The forms of the lenses and the curvatures evidently are, as far as the achromatic
lenses are concerned, dependent on the materials employed for achromatizing them.
We indicate further on the curvatures of an eye piece constructed with the following
materials:

page_text: of all the others owing to the presence of the longitudinal grooves, while the helicoidal
grooves on the other hand compel the glasses to be conveniently displaced in the
direction of the axis of the system.
Figures 4 to 7 represent a pancratic eye piece, in which 02 and 05 are achromatic
and in which d¹ and d² vary; Figures 4, 5 and 6 representing the separate parts
constituting the eye piece, and Figure 7 the instrument complete. The last three
glasses 03, 04 and 05 are rigidly mounted in the drum T (Figure 4) entering with
friction into the tube A (Figures 4 and 5), carrying a milled knob M. The drum is
secured in its position by the bell shaped piece C, which is provided with a lug
and adapted to be screwed on to the front end of the tube A. Into the interior
of the latter enters under very slight friction the tube holding the righting device
or rectifier B (Figures 4 and 5), the said friction being obtained by means of the
small inner projections p p. The tube which holds the rectifier is provided with
longitudinal grooves along two opposite generatrices. Into these grooves enter
the extremities of the screws vv fixed on the tube A.
Finally the two tubes are placed in the interior of a third tube E (Figure 6), on
which two sets or pairs of helicoidal grooves h¹ h¹ h2 h² are provided each one of
which extends over one half of a circumference. The heads of the screws V1 y1 y2 y2
enter these grooves.
Small outer projections p p ensure smoothness of friction of
the tubes A and B in the interior of E. Lastly a thin tube (not shown) in the
drawing encloses the tube E and serves to protect the interior of the eye piece from
light or dust that might find its way through the grooves.
As the milled knob M is acted upon, with the object to turn the tube A, the
longitudinal grooves permit of a reliable transmission of the movement to the tube B
being obtained, and the helicoidal grooves compel the two tubes A and B and conse-
quently the glasses which are firmly mounted in each of them, to be properly
displaced in the direction of the axis.
In order to obtain the outlines of the grooves we proceed in the manner described
with reference to Figure 8.
Supposing A B C D to be a front view and profile of that portion of the cylinder,
upon which the grooves are to be produced. As we have seen, O¹ is fixed, its
position consequently will correspond to the right A B perpendicular to the
generatrices. For the groove h² serving to guide 02 we take any arbitrary form, for
instance a regular helix. It is evident that by means of the table above given, one
may construct in dots or points a second groove h¹, serving to guide 03 01 and 05.
The representation indicates the outline of the grooves of a pancratic eye piece with
five.achromatic lenses.
Figure 8 indicates the construction, supposing F1
= 50.

page_text: The
concerned, it is absolutely similar to that which has just been described.
grooves h¹h h¹ always guide the tube A, to which are rigidly connected O¹ and
05, the distance d of which does not vary. For the purpose of displacing 0³, this
lens is mounted on a drum T¹ T" different from (TT) which carries the two eye.
glasses. This drum T T and consequently 03 are guided by two screws V3 V³, the
heads of which engage with two helicoidal grooves h³ ³ ³ h³. Each of these
screws, on the other hand, traverse a longitudinal groove 12 12, provided in the tube A.
The tube A, adapted to be turned by means of the milled knob M actuates 0³ as it
rotates and the screws V3 VS following along the outline of the grooves h³ h³, forcibly
impart to it the proper movement of translation relatively to the fixed glass O¹
In the same manner as we have done in the above case, we indicate in the repre-
sentation, Figure 8ª, the outlines of the grooves of a pancratic eye piece having five
Dated this 29th day of July 1897.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicants.