Improvements in Photographic Lenses.
Year: 1895

page_text: PROVISIONAL SPECIFICATION.
Improvements in Photographic Lenses.
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1, THOMAS RUDOLPHUS DALLMETER, of 25 Newman Street, in the County
of Middlesex, do hereby declare the nature of this invention to be as follows:—
In older types of lenses, especially portrait lenses, in which the character of the
glasses employed necessitated a higher dispersive power to accompany a higher
refractive index (a condition no longer necessarily existing since the invention of
the Jena glasses) flatness of field always involved astigmatic aberration or if the
astigmatic error was practically eliminated the curvature of field was considerable.
The construction of the best forms of the older types has been the most skilful
compromise between these errors.
+
The object of this invention is to remedy this defect by an astigmatic corrector
which eliminates the residual errors (of astigmatism and curvature of field) of the
older forms of positive lenses, largely increasing their covering power at the expense
of an inconsiderable lengthening of the focus.
For this purpose I place behind any of the old forms of lenses a negative
combination consisting of a double concave lens cemented to a double convex lens
of greater dispersive material and higher refractive index.
•
Dated this 23rd day of December 1895.
Improvements in Photographic Lenses.
I THOMAS RUDOLPHUS DALLMEYER, of 25, Newman Street, in the County of
Middlesex, 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 :-
In older types of lenses, especially portrait lenses, in which the character of the
glasses employed necessitated a higher dispersive power to accompany a higher
refractive index (a condition no longer necessarily existing since the invention of
the Jena glasses) flatness of field always involved astigmatic aberration or if the
astigmatic error was practically eliminated the curvature of field was considerable.
The construction of the best forms of the older types has been the most skilful
compromise between these errors.
The object of this invention is to remedy this defect by an astigmatic corrector
which eliminates the residual errors (of astigmatism and curvature of field) of the
older forms of positive lenses, largely increasing their covering power at the
expense of an inconsiderable lengthening of the focus.
For this purpose I place behind any particular form of the old types of lenses
after having determined the residual errors referred to, my astigmatic corrector
which is calculated to eliminate these errors. The form necessary to do this in
the case of Petzval's construction consists of a negative combination consisting of
a double concave lens cemented to a double convex lens of greater dispersive
material and higher refractive index.

page_text: This reversal of material applies in every case, but the shapes of the lenses may
be varied for some of the older types of lenses.
The drawing shows my invention applied to the well known Petzval lens made
up of four lenses A. B. C and D.
The lenses E and F shown in dotted lines form my astigmatic corrector.
The radii of curvature of the lenses are as follows:-
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed 1 declare that what I 2
claim is :-
1. The combination with the older forms of positive lenses of a negative
compound lens consisting of a negative lens and a positive lens of greater
dispersive material and higher refractive index substantially as described.
2. The combination with the older forms of positive lenses of a negative 2
compound lens consisting of a double concave lens and a lens of greater dispersive
material and higher refractive index substantially as described.
3. Photographic lenses substantially as described and shewn in the drawing.
Dated this 10th day of October 1896.