PROCEEDINGS OF THE SOCIETY. 
423 
and the micrometer, attached to a Microscope and lamp, was also handed 
round for inspection. 
The Chairman thought that papers like the one just read were of 
great practical value, and that all would' be grateful for the observa= 
tions which had been made. 
Mr. Thomas Comber’s paper “On a Simple Form of Heliostat and 
its application to Photomicrography,” was read by Mr. Mayall, who 
explained that having been much struck by the excellence of the results 
of Mr. Comber’s work, shown at the last meeting, he had requested him 
to forward a description explaining the construction and application of 
his heliostat. Mr. Comber had not only given these explanations, but 
had sent the heliostat for inspection, together with photographs showing 
the installation of his photomicrographic apparatus. Apart from the 
question of the extreme simplicity of the heliostat, which was mainly 
due to limiting the reflection of the mirror to the polar direction, and 
deflecting the pencil in the horizontal direction in the axis of the 
Microscope by means of a fixed mirror, placed at half the angle of the 
latitude, above the heliostat mirror, Mr. Comber had rendered important 
service to photomicrography, by showing how the heliostat might be 
placed close to the Microscope, so that the error due to slight inaccuracy 
of the adjustment of the heliostat might escape the optical leverage 
which took place when the reflected beam was made to travel through 
a considerable space — which obtained with heliostats, as usually placed 
with reference to the Microscope. Mr. Comber’s observations on 
the fallacy of employing monochromatic illumination for photomicro- 
graphy would have to be most carefully considered by microscopists, 
for if they stood the test of experience — and Mr. Comber was evidently 
a careful observer — the process would be permanently simplified. Mr. 
Comber had certainly devised a very practical method of using sunlight ; 
he wished he could give them any equally practical means of obtaining 
more of it. 
Mr. E. M. Nelson said that with regard to Mr. Comber’s paper, 
which he had listened to with great interest, there were one or two 
points upon which he would remark. He thought great credit must be 
given to Mr. Comber for the admirable way in which he had simplified 
the heliostat. However necessary a universal heliostat might be for 
other scientific purposes, the instrument exhibited furnished a practical 
demonstration that for photomicrography not only was it unnecessary, 
but the other and far simpler and less expensive non-universal was 
really the more efficient of the two. The universal heliostat gave a 
steady beam over a considerable range of directions, but the non- 
universal only in one. On that account the non-universal needed two 
mirrors ; the slight loss of light thus occasioned was no real detriment, 
because, generally speaking, there was more light than was required. 
Then, with reference to monochromatic illumination, he would direct 
special attention to that passage where Mr. Comber stated that the 
plate itself made a time selection of the actinic ray. The importance of 
that sentence could not be over-estimated. Speaking for himself, he 
could only look back with regret at the amount of time wasted with 
prisms and absorption media, merely from the want of knowledge of 
