PROCEEDINGS OF THE SOCIETY. 
139 
some orange rays were also transmitted. Notwithstanding this defect 
the screen was an excellent one for visual purposes. In order that these 
glasses might be accurately matched at any time, he appendod the 
following tintometer values : — 
Green glass = 6*2 Green -f 10 • 3 Blue + 1*4 Excess of light. 
Blue glass = *46 Violet — (— 5 * 94 Blue. 
Green and Blue glass = 2*7 Green -f- 16 * 3 Blue. 
Mr. Lovibond had lately sent him a new screen composed of methy- 
len-blue dissolved in water. A few granules of the powder of this 
highly concentrated dye were sufficient for one of the ordinary light 
filter bottles. This screen gave a broad dark band in the red, orange, 
yellow, and yellow-green portions of the spectrum, cutting them all out, 
with the exception of a small portion at the lower end of the red. 
Visually, it was a blue screen, the red being too weak to affect the eye, 
except when an exceedingly delicate test was used with an achromatic 
lens having a very red correction. 
Photographically, the red had absolutely no effect, and as the screen 
passed freely those rays belonging to the higher portions of the spectrum, 
exposures were not unduly lengthened with ordinary plates. 
The value of this screen consisted in the suppression of the yellow- 
green, yellow, orange and brightest part of the red, while the pure 
green, blue-green and blue were transmitted at almost full intensity. 
Mr. Nelson felt sure that these excellent qualities possessed by 
Mr. Lovibond’s screen would be appreciated by microscopists in 
general. 
With regard to the density of the screen, he said in conclusion that 
it was better to err on the side of too light rather than of too dark a 
screen. 
The President — in moving a vote of thanks to Mr. Nelson for his 
communications — said that the Microscope exhibited was one which 
might no doubt be very useful for the purpose for which it was designed, 
but it struck him that if it was to be used very much, some of the parts 
would require much more steady fixing than they seemed at present to 
possess ; as it was, the reflecting mirror would almost fall out of position 
by its own weight and seemed very much in need of some means of 
clamping it. He thought also that it might be difficult to get the light 
in such a direction that the shadow of the objective would not fall upon 
the mirror if anything like a short focus was required. 
Mr. Nelson said that all the movable fittings were sprung so that 
they could very easily be tightened up to any extent required to take 
up the wear. The Microscope was intended only for the examination 
of large objects with low powers, so that very short focus objectives 
would never be used. 
Mr. Nelson said he had been prevented from attending the last 
meeting, otherwise he had intended pointing out several errors in the 
statements of facts made by Dr. J. B. Nias in his paper on the Conti- 
nental form of Microscope.* He said it was not his intention to criticize 
* See this Journal, 1893, p. 596. 
