110 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
photomicro graphic work, so that an exposure of three hours had to be 
given. 
When developed the plate showed a very satisfactory reproduction of 
the colours of the original. 
Simple Method of taking Photomicrographs of Opaque Objects.* 
— Drs. E. W. Carlier and G. Mann have sought to obtain photomicro- 
graphs of the surface instead of sections of various animal and vegetable 
tissues. 
The authors give a detailed description of the apparatus and process 
which they employed. 
A horizontal camera made by Mr. Forgan, of Edinburgh, fitted to a 
large Zeiss Microscope, was used. The preparation was illuminated 
with a beam of light concentrated by a bull’s-eye condenser. The light 
of an argand burner was found to be sufficient in most cases for focusing 
on the ground-glass screen. Magnesium ribbon fed through a piece of 
brass tubing was used for taking the photograph. 
Ilford ordinary medium isochromatic plates were used. For timing 
the exposure an ordinary metronome set to half seconds was found to be 
very convenient. 
As developer the old Ilford hydrokinone formula was used. After 
developing, the plates were immersed in an acetic acid bath, as by this 
process the negatives were rendered much clearer. 
The best results were obtained with most objects when the incident 
light formed with the plane of the stage an angle of about 40°. Only 
low-power lenses such as No. 1 and 3 of Leitz were used. 
Gelatino-chloride papers of the kind termed “ Solie ” (Eastman) 
were used for printing, and Eastman’s combination toning and fixing 
bath for toning. 
(5) Microscopical, Optics and Manipulation. 
On the Passage of Light through a System of Spherical Lenses.y 
— Mr. C. V. L. Charlier considers that the theory of the spherical aber- 
ration of light, although of the utmost importance in the construction of 
optical instruments, has not by any means been sufficiently developed to 
answer the requirements of practical optics. As contributions to the 
subject, the author intends to publish investigations : — 
(1) On the image of a point by the passage of light through any 
number of centric lenses. 
(2) On the choice of the radii of curvature, refractive indices, thick- 
ness and distances apart of the lenses, in order to obtain the best pos- 
sible images. 
(3) On the photographic images of stars. 
As a preliminary to these investigations, in the present paper he 
gives the mathematical determination of the equation for the aberration 
curve as defined below. 
A system of lenses is considered with refracting surfaces spherical, 
and with centres on one straight line, the axis of the system. Through 
the centre of curvature of the first of these surfaces a plane, called the 
* Proc. Scottish Micr. Soc , 1893-4, pp. 115-21. 
+ Nova Acta Sci. Upsala, xvi. (1893) viii. 
