Improvements in or connected with Optical Projection Apparatus.
Year: 1898

page_text: Date of Application, 17th Feb., 1898-Accepted, 2nd Apr., 1898
COMPLETE SPECIFICATION.
Improvements in or connected with Optical Projection Apparatus.
I, FRIEDRICH MÜLLER, of 32, Ohmstrasse, Cologne-on-the-Rhine, in the Empire
of Germany, Manufacturer, 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:―
The ordinary optical projection apparatuses, such as magic lanterns and the like,
the principle of which has also recently been employed for the construction of so-called
advertising lanterns, rests on the property of convex lenses of projecting an enlarged
picture of an article removed somewhat more than the focal distance, which picture
is received on a white screen or opaque disc. The pictures to be projected are formed
of transparent pictures or photographs placed on glass, which are clearly illuminated
by a powerful source of light.
A serious difficulty in the pictures produced by means of convex lenses consists in
the so-called spherical aberration or departure from the spherical form of the
refracting surfaces. In consequence of this aberration, the rays proceeding from one
point of the object are not combined precisely at one point but over a spherical focal
surface, whereby the picture projected on a smooth surface or an opaque disc is not
clear, more particularly near the edges of the field of projection, and shows strongly
disappearing and broken outlines.
By the use of an aplanatic objective arrangement in which the spherical deviation
is considerably diminished, the drawback may be somewhat diminished but not
entirely removed; but by the use of transparent representations placed on spherical
glass discs, pictures of equal sharpness all over may be obtained, of course apart from
the chromatic deviation which can only be avoided by the use of achromatic double
lenses.
The semi-diameter of curve of the spherical glass discs is directed towards the focal
distance of the series of lenses employed and according to the degree of enlargement
of the picture projected.
This invention has for its object therefore an improved system of projection by the
use of transparent pictures or photographs painted or produced on spherical glass
discs.
In the accompanying drawings-
Figure 1 shows a diagram of the principle of a projecting apparatus when used
with pictures painted on a straight glass disc, a being the source of light, b the
transparent picture, c the lens, d the white screen or opaque disc on which the
picture which is not sharp enough at the edges is projected, whilst e indicates
the surface on which a sharp picture would appear. Thus according to Figure 1, if
a spherical opaque disc were employed as the projection surface, an evenly sharp
picture would be obtained. In practice, however, this form of construction is not
applicable, because the manufacture of exactly spherical glass discs of more than
50 centimetres diameter is too difficult. Figure 2 shows the object of the invention also
diagrammatically. 61 in this case indicates the transparent picture of a spherical
disc and d the surface on which an equally sharp enlarged picture must appear. It

page_text: will thus be seen that the proper effect is obtained by the use of this spherical
disc b¹.
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 :-
In projection apparatus, such as magic lanterns, advertising lanterns and the like,
the use of transparent pictures or photographs painted or produced on spherical glass
discs, for the purpose of avoiding spherical aberration and producing an equally sharp
picture, substantially as hereinbefore described and shown.
Dated this 14th day of February 1898.
WM. P. THOMPSON & Co.,
Of 6, Lord Street, Liverpool, Agents for the Applicant.