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PROCEEDINGS OF THE SOCIETY. 
Mr. Conrad Beck inquired if Mr. Nelson bad carefully measured 
tlie amount of distortion obtained with a grating eye-piece when the 
drawing was made on a plane surface instead of a curved surface ? 
Mr. Nelson said be bad been doing that, and hoped at another time 
to give the results. It varied, of course, according to the size of the 
picture, and amounted in a 3-in. picture to 1/10 in. ; but in a 2 in. 
picture it was very small indeed, being no more than *0033 in. 
The President said that no doubt Mr. Nelson bad pointed out a very 
sensible source of error in drawing with ruled squares, but he thought 
they were all fully aware that no method was known which was entirely 
free from error. He was himself still of opinion that ruled squares in 
the eye-piece gave about the best results of any method, and he fancied 
the eye more quickly recognized any error with squares and was there- 
fore more likely to correct it, because in any such drawing there was 
always a margin left for correction. No doubt Mr. Nelson was perfectly 
correct in saying that absolutely accurate drawings were not to be made 
by any method known to them at present. 
Mr. J. W. Gifford read a paper on “ Photography by Monochromatic 
Violet Light,” illustrating the subject by lantern photographs shown 
upon the screen. 
Mr. E. M. Nelson could not say much about this paper until he had 
an opportunity of reading it, but he thought the subject was a most 
interesting one. He looked to the use of screens as a means likely 
to lead to great developments in photomicrography, the effect of their 
use being to cause the lens to act as if it was better corrected, and to 
sharpen up the image wonderfully, so that a proper screen really gave 
the means of making a violently coloured lens equal, for photographic 
purposes, to an apochromatic. It was said that they enabled the ultra- 
violet rays to be used, but for his own part he did not believe that 
they got through at all. Mr. Gifford had given them a most interesting 
paper, and he should be very glad to have the opportunity of reading it. 
Dr. W. H. Dallinger said he had used screens for a long time with 
the greatest pleasure and the fullest profit, and was quite certain that in 
the future, as their value became more generally recognized, they would 
be much more generally used. He should not be at all surprised to find 
during the next ten years that they had reason to be much indebted 
to Mr. Gifford for working out the subject. If they could so simply get 
monochromatic light, they had at hand a ready means of making bad 
lenses into good ones and good lenses into better. 
Mr. Beck said that any one used to making objectives was aware 
that all final corrections had to be done by visual tests in the Micro- 
scope, and realizing as he did how extremely difficult it was to see these 
deep violet rays at all, it was almost too much to expect opticians to 
correct the violet lines alone. It was so difficult to see these rays that 
it was hardly possible to get perfect corrections for them. 
Mr. Gifford had thought the same thing when he began to work at 
this subject, but he had s'nce found that it w r as not so difficult after 
all. He used a 3-in. quartz condensing lens, but found there was quite 
enough light to see to focus by, even when a magnification of x 1000 
