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PROCEEDINGS OF THE SOCIETY. 
that fact. His was no isolated experience. With reference to the 
correction of achromatic lenses for photomicrography, he went some time 
ago through an exhaustive series of trials with an achromatic homo- 
geneous-immersion objective, whose actinic focus was displaced from its 
visual, but which, nevertheless, yielded a very fine image at that visual 
focus. A number of biconvex correcting lenses, placed immediately 
behind the objective, were tried, and it was found that a suitable lens 
would bring back the actinic focus to the plane of the visual. When, 
however, the focus was brought back, the actinic image was quite unlike 
the visual, inasmuch as it had lost all sharpness. The image resembled 
that yielded by an objective quite out of adjustment. He had next 
tried monochromatic illumination, with no better results. One thing 
only remained to complete the experiments, and that was monochro- 
matic sunlight. Not being in a position to carry out these experiments 
himself, Mr. Comber very kindly undertook them for him. The results 
he obtained with a very fine duplex-fronted water-immersion 1/12 of 
1*22 N.A. entirely agreed with those formerly obtained. In his 
opinion, no ordinary achromatic lens could be corrected for photo- 
micrography. He did not for a moment doubt that its actinic focus could 
be brought into coincidence with its visual ; but if the lens yielded a 
crisp visual image its actinic image would be out of correction. He 
believed that no lens could be said to be corrected for photomicro- 
graphy which was not apochromatic in the strict meaning of the word. 
The Chairman said that this heliostat seemed to be rigidly for use 
in one latitude only. The subject was one of much interest and utility 
to those who were working at photomicrography, and their thanks were 
jwstly due to Mr. Comber for his communication. 
The following Instruments, Objects, &c., were exhibited:— 
Mr. T. Comber : — Heliostat in illustration of his paper. 
Mr. J. Mayall, Jun. : — Marzoli’s Achromatic Lens (1808). 
Mr. E. M. Nelson: — New form of Micrometer Eye-piece. 
Mr. Vallance : — Eye-piece with Compound Eye-lens giving extra 
large field. 
New Fellows: — The following were elected Ordinary Fellows: — 
Messrs Philip Braham, F.C.S., William Forgan, Thomas H. Hall, and 
W. Scott Lang, M.D. 
