PROCEEDINGS OF THE SOCIETY. 
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either with or without a yellow screen ; but the image will be likely to 
be found more or less divergent from that on the camera screen when 
plates are used not colour-correct. 
The fact that certain rays are made active at the visual plane of the 
objective, when isochromatic plates are used, is due solely to such plates 
and not to any screen or ray- filter interposed between them. 
That I find no evidence of any such refinements of illumination and 
correction being required to produce a good photographic image as stated 
by Dr. Piffard.” 
Mr. Nelson said he should like to ask Mr. Smith if he had tried 
different kinds of lights in making his experiments ; had he tried 
coloured lamplight or daylight ? 
Mr. Smith said he had used the light of a paraffin lamp in every 
case. 
Mr. Nelson thought the results obtained were very remarkable and 
extremely valuable, and he quite endorsed the opinion expressed, but he 
thought if Mr. Smith had employed sunlight he might perhaps have 
found that the isochromatic plate would not then select a ray which was 
sufficiently near to produce quite the same effect. With a paraffin lamp 
they had a light which, though fairly white, had still a much larger pro- 
portion of yellow than was found in sunlight. 
Mr. C. Haughton Gill said he could only repeat the observations he 
made on the former occasion when this subject was before them. Given 
ordinary isochromatic plates and ordinary objectives the explanation was, 
not that the plates had corrected the lenses for differences of foci, but 
simply that such plates were comparatively insensible to the blue and 
violet rays, but were as sensible to the others. Both lenses and plates 
being corrected for the yellow — or the visible image — the plate simply 
made use of the rays which fell upon it, amongst which the blue rays 
were not sufficiently strong to affect the silver on the plate. 
Prof. Bell said they had received some photographs and a letter 
from Dr. H. G. Piffard, of New York, bearing upon the same subject, 
which he read to the meeting, as follows : — 
“I have received the December number of the Journal, and note 
the appearance of the letter which I sent you in May last, together 
with comments thereon. I did not at the time deem it necessary to send 
photographs illustrating experiments which could readily be repeated 
by any one possessing objectives whose corrections approximated those 
described, for I fancied many such could be found in London, especially 
among those of older make. 
The order given to Spencer * for a 1/6 to be specially corrected 
for D light was filled, and I enclose a few photographs made with it. 
The lens is a homogeneous immersion of N.A. 1*35. Spencer has also 
made a 1/15 on the same system for a friend of the writer, and I 
enclose one photograph made with it ( AmpMpleura LindJieimerii). The 
visual performance of the lenses is superb. In these two lenses I find 
the spherical aberration nil, or if any exists it is too slight to enable me 
to determine whether positive or negative. The colour-aberration is 
negative — slight. A 1/4, more recently made, was brought me by a 
* The firm named is Spencer and Smith, Buffalo, N.Y. 
