E. M. XELSON ON METHODS OF ILLUMINATION. 297 



if an insect preparation is viewed through a screen which cuts 

 out all below the yellow, the orange-yellow colours, peculiar 

 to that class of objects, are darkened out of all recognition. 

 This is just where the art comes in of constructing a screen 

 which shall remove every thing objectionable and leave that 

 which is necessary for the proper observation of the object. The 

 fact is that our heads were swelled by the " table of resolving 

 powers " published on the cover of the R. M. S. Journal, where 

 the three selected lights had wave lengths of 5,269 for visual, 

 line F 4,861 for screen, and -ijOOO for photography.* I altered 

 the visual to Gilford's maximum 5,607, and photography to 

 4,341 ; this last should be brought still lower down the spectrum 

 to the photographic maximum through glass, of 4,603, and 

 the screen placed at least as low down as b, or 5,184, if not 

 lower. 



Gifford's screen is formed by placing a piece of worked-down 

 peacock-green glass in glycerine coloured by malachite green. 

 The solution may be made so strong that a very shallow trough 

 is sufficient. A disc of peacock-green glass may be edged so as 

 to fit in the carrier below the substage condenser ; a cell, made 

 by a metal ring, filled by the solution and covered by a thin 

 covering-glass in the usual way, precisely like a preparation in 

 a fluid mounted slide. Screens mounted in this manner are very 

 delicate, and may in time leak. I used them extensively, but 

 aftei'wards placed the fluid and the glass in a Leybold's cell, 

 0*2 in. thick, on a separate stand. 



Dr. Miethe's screen consists of a saturated solution of acetate 

 of copper in a Leybold's cell, 0*8 in. thick. To this I added an 

 interior trough, 0*2 in. thick, filled with water coloured by 

 methyl blue. This screen when used with lamplight should be 

 thicker, and the water bluer, than for use with daylight. The 

 poisonous and corrosive nature of acetate of copper is the great 

 disadvantage of this screen, which, notwithstanding its excellent 



* J. R. M. S. 1885, p. 972 where the photographic resolving limit is put 

 at 127,000 lines for N.A. 1-0 ! 



