COLOUR SCREENS 155 



intensity in the transmitted part. There is always some 

 absorption of light in the transmission bands, and the aim 

 of the screen-maker is to make that loss as small as 

 possible , consistent with a sharply defined band and total 

 absorption in the other parts' of the spectrum. The 

 majority of filters do not transmit monochromatic light, 

 but a band extending over a considerable length of the 

 spectrum, or several such bands, and it is impossible to 

 tell merely by inspection whether a screen is mono- 

 chromatic, but the required information is very readily 

 obtained by examining the transmitted light by means 

 of a direct-vision spectroscope. 



Material of Screens. Screens may be made of coloured 

 glass, dyed gelatine either unmounted or mounted on 

 glass, or of solutions contained in glass cells. Of these 

 ordinary glass is liable to refract the light that passes 

 through it more or less unevenly, and on that account its 

 use is to be deprecated for critical work, as it is no good 

 taking every care that lenses and condensers should be 

 free from aberrations, and then introducing these with 

 the screens. For low- and medium -power and even high- 

 power work, when high resolution is not required, glass 

 may be used without fear, if it is of reasonably good 

 quality and parallel-sided. The same remarks apply to 

 the use of ordinary glass for making the troughs for 

 liquids. 



Glass flats are now made in various colours, and, as 

 their surfaces are as carefully worked as those of a lens, 

 they do not introduce these errors, and form the best 

 material for colour screens and the sides of glass vessels ; 

 but unfortunately their cost is in many cases prohibitive, 

 and it is difficult to get a series with the desired absorp- 

 tions. Dyed gelatine forms an excellent substitute for 

 glass, so far as efficiency and freedom from errors are con- 

 cerned, but with the disadvantage that screens made of 

 it are very liable to injury. Gelatine, however, has the 



