MONOCHROMATIC ILLUMINATION 



321 



down the body, so that the objective on the nose-piece nearly 

 touches the crystal ; then focus with the draw-tube exclusively. The 

 suit-stage condenser should be racked up close to the under side of 

 the crystal. 



The use of monochromatic light is frequently desirable in micro- 

 scopic work, especially blue light, although of less moment than 

 in p re-achromatic days. The usual method of obtaining coloured 

 light is to pass sunlight through coloured glass, or through a 

 coloured solution, such as the ammonio-sulphate of copper ; but this 

 is a most imperfect and unsatisfactory method, and does not give 

 monochromatic light. This most valuable mode of illumination has 

 been made possible by the use of what is now known as the Gifford 



screen, from the name of its 

 inventor, Mr. J. \V. (Jittnrd ; 

 and when artificial light is 

 used one of these screens 

 should be interposed between 

 the lamp and the sub-stage 

 condenser. It is shown in fig. 

 266, and consists of a glass 

 trough, about 3 inches long by 

 2 inches broad and y^ths deep, 

 filled with a solution of methyl 

 and glycerin mixed 



FIG. 266. Gifford screen with an adjustable 

 stand. 



FIG. 2(57. Gifford' sF-line mono- 

 chromatic light screen. 



warm. Now this solution passes a little band of infra red, which 

 must be cut out. To do this a piece of signal green glass just fitting 

 the trough is placed in it. 



A piece of ordinary commercial signal green would cut out too 

 much light, and render the screen too opaque; therefore it is 

 requisite to have this signal green glass worked down to about half 

 its thickness, so that only the infra red passed by the methyl 

 green is cut out, and nothing more. This screen is called an 

 F-line screen, because the F line is in the centre of the band passed 

 by it. The band for general microscopical purposes may usefully 

 extend from E to . "The importance of this screen cannot be held 

 too high by the modern microscopist. It makes semi-apochromatic 



