Improvements in or relating to Optical "Erecting" Apparatus.
Year: 1897

page_text: Date of Application, 20th July, 1897--Accepted, 21st Aug., 1897
COMPLETE SPECIFICATION.
Improvements in or relating to Optical "Erecting" Apparatus.
I, CHARLES SHELDON HASTINGS of 248 Bradley Street, New Haven, Connecticut,.
Professor of Physics, Yale University 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 :—
This invention relates to a new and improved optical appliance, to which the name
erecting prism has been applied, and which is designed to change any optical image,
real or imaginary, 180° in its own plane without material distortion or lateral
displacement.
This said invention consists in certain new arrangements and combinations of
reflecting surfaces or prisms, as will hereinafter more fully appear, the novel features
being pointed out in the claims.
In the accompanying drawings illustrating the preferred form of embodiment of the
said invention :
Figure 1 is a side elevation and
Figure 2 an end view of an old form of erecting prism.
Figure 3 is a side elevation and
Figure 4 an end view of a modified and improved construction of the prism,
Figure 1..
Figure 5 is a side elevation.
Figure 6 is an end view and
Figure 7 is a plan view of the new inverting or erecting prism..
Similar figures and letters of reference in the several figures indicate the same
parts.
One of the principal and most important applications of the new erecting prism is
in connection with telescopic instruments, as a substitute for the inverting lens or
lenses heretofore commonly employed, for converting telescopes, large or small, inte
erecting (terrestrial) telescopes. As is well known the most perfect refracting tele-
scope which has heretofore been constructed presents to the eye of the observer an
inverted image. Since this is immaterial in an astronomical use where everything
should be sacrificed to perfection of definition, such telescopes, which moreover are the
simplest in construction, are called astronomical telescopes. But inverted aspects of
terrestrial objects are ordinarily not acceptable to any except the very small group of
philosophers who have succeeded in emancipating themselves from the purely arbitrary
conceptions of up and down, hence it is usual to employ some means for correcting
this perverted condition of affairs as it appears to the majority. There are two methods
in general use for modifying the telescope to meet the wants of the larger portion of
mankind not included in the class defined above; namely by using (a) a negative, 2.e.,
essentially concave, lens at the proper distance for distinct vision inside the focus of
the telescopic objective, or (b) by replacing the simple ocular of the astronomical
telescope by a more complex one, which is in fact a compound microscope and thus
yields an inverted view of the inverted image.
The first named type is known as the Galilian telescope and is universally familiar

page_text: in its use in opera-glasses. The second type is known as the terrestrial telescope or
spy glass. The objection to type (a) is the smallness of the field of view, both
absolute and apparent, which they possess. This fault is so serious that only very
low magnifying powers are generally practicable, say up to 5 or 6 diameters.
The objections to type (b) are two, first, a very material reduction in optical
definition; and second, the inordinate increase in length, unless relatively high powers
be employed.
There is an entirely distinct method of inverting the image, namely, by oblique
reflections from plane surfaces. This has been accomplished by the employment of
two polyhedra cemented together, as represented in Figures 1 and 2, the rays of light
entering at a being reflected by the surfaces b, c, d, and c, and emerging at ƒ.
On account of the technical difficulties in making this prism the improved form
represented in Figures 3 and 4, is to be preferred, it being constructed of three
rectangular prisms cemented together, two of said prisms being of like dimensions,
and the third larger as represented in the drawings.
There are general objections to all of these forms of inverting prisms, namely, their
width and the fact that the image is displaced laterally by a distance depending upon
the size of the erecting prisin. This last named property is especially inconvenient
when the prisins are to be used in binocular telescopes. The new type of erecting
or inverting prisms illustrated in Figures 5, 6 and 7, obviate the defects mentioned.
In the preferred form shown it may be said to comprise three prisms, 1, 2 and 3,
furnishing the four reflecting surfaces, g, h, i and j. The prism 2 is a right angle
prism, that is to say, its reflecting surfaces h, i, are at an angle of 90°, while the angles
of prisms 1 and 3 admit of a wide range of variation, but for most purposes the 30°, 60°
and 90° shown, will be found preferable.
The prisms 1 and 3 are crected one behind the other and extend equally on
opposite sides of the line of intersection of the two planes forming the reflecting
surfaces h, i, of prism 2, so that the rays of light entering prism 1 will emerge from
prism 3, in inverted position but without lateral displacement beyond such as is due
to the inversion of the image. This is indicated by the dotted lines in Figures 5
and 7, representing the course of a single ray of light traversing the prism.
Having determined upon the angles and dimensions of prisms 1 and 3, the dimen-
sions of prism 2 are determined by considerations which would at once be obvious to
any skilled optician. Thus under the conditions illustrated, the prism 2 would be of
the dimensions indicated by dotted lines in Figure 4, in which case the prisms 1 and 3
might be cemented directly to the upper surface of prism 2, but as this would involve
an unnecessary and useless amount and width of material comprising the right angle
prism 2, the excess at the sides included in dotted lines, may be removed or omitted;
so too, the end sections of prisms 2, included in dotted lines are unnecessary and may
be omitted; and all that part intermediate the triangular section bounded by full
lines indicating prism 2 proper, and the two prisms 1 and 3, may be omitted without
departing from the spirit of the present invention; but it is preferred, for obvious
reasons, that this intermediate space should be filled so as to eliminate reflecting
surfaces, hence it is preferred to make the prism 2 smaller, that is proportionally, as
indicated in the drawings, and fill the interval between it and the prisms 1 and 3 with
a parallel sided plate 4,
As to prism 2 it may be said, that the right angle must be very accurate, say to
within one minute of arc, but the other angles admit of errors vastly greater than this
without material injury to the performance.
Although in the preferred construction the new erecting prism is made up of four
pieces of glass cemented together, it is perfectly evident that whether made in one,
two, or more parts, so long as the four reflecting surfaces are caused to occupy substan-
tially the relations indicated, its performance will be substantially the same. Thus, as
hereinbefore explained, the three prisms 1, 2 and 3 may be employed alone or in
combination with the parallel sided plate 4, prism 2 and plate 4 may be combined as
one piece of glass, or one or both of the prismns 1 and 3 may be formed integral with
plate 4; prism 1 or 3, a portion of plate 4 and the whole of prism 2 might be made

page_text: from one piece of glass, and the other prisms 1 or 3 and the remainder of plate 4, of
another; finally the whole might be formed of a single piece of glass, although there
would, in such case, be great technical difficulties in construction experienced. It may
be found advantageous to cover all the reflecting surfaces with a coating of silver, or
other reflecting material, to protect them fro:n soiling, etc.
It has not been considered necessary to illustrate the manner of applying the new
erecting prism to a telescope, as this will be readily understood by any one skilled in
the art, nor is it to be understood that the new prism is limited in use to such
instrument, as it is obvious that it could be employed to advantage in other
instruments and under different conditions, as for example, in dissecting
inicroscopes.
Upon comparison with the older forms of erecting prisms illustrated in Figures 1
to 4 inclusive, it will be noticed that the new prism presents this point of great
importance and advantage, it preserves the axial alignment of the lenses employed,
and it does not effect lateral displacement of the object as viewed in the ocular. This
is due to the fact that the two reflecting prisms 1 and 3 are arranged and disposed
longitudinally of prism 2, with their reflecting surfaces y and j, extending across both
reflecting surfaces, h, i, of said prism 2, so that the ray of light reflected by surface g
or j on one side of the median line will be directed against surface h, and be reflected
laterally against surface i, while the rays reflected on the opposite side of the median
line will be directed against surface i, and be reflected laterally onto surface h, the
rays crossing within prism 2, after reflection from g or j, and finally emerging with a
lateral displacement just proper to inversion of image only, and which therefore
vanishes for an axial ray.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is
1. An erecting system or apparatus, such as described, the same comprising four
reflecting surfaces, of which the intermediate surfaces are two planes intersecting at
an angle of substantially 90° and the first and fourth surfaces are arranged one behind
the other, in the direction of movement of light through the system with the line of
intersection of their planes substantially at right angles to the 'line of intersection of
the intermediate surfaces, said first and fourth surfaces extending across and on
opposite sides of the line of intersection of the interinediate planes; substantially as
described.
2. An erecting system or prism, such as described, the same comprising four
reflecting surfaces, g, h, i and j, of which the intermediate surfaces h and i are
arranged on intersecting planes, and the first and fourth surfaces gand j are arranged
in line longitudinally of said surfaces h and i, said surfaces g and j, constituting con-
verging planes lying transverse to and extending on opposite sides of the line of
intersection between said surfaces h and i.
3. An erecting system or prism, such as described, combining in its structure, a
prisin 2, and two prisms 1 and 3, the latter arranged in line longitudinally of prism 2,
with their reflecting surfaces extending laterally across both reflecting surfaces of
said prism 2.
4. The combination to form an erecting prism such as described, of the two
reflecting prisms 1 and 3, located one in front of the other, a right angle prism 2,
and a parallel sided plate 4, interposed between said prisms 1, 3 and prism 2, the
latter arranged with the line of juncture of its reflecting surfaces in a plane bi-secting
both prisins 1 and 3; substantially as described.
Dated this 20th day of July 1897
BOULT & WADE,
Agents for the Applicants.