COLOUR SCREENS 157 



solution, 1 part dye in 1000 parts water, until it is judged 

 to be dark enough, then rinsed in water and left to dry. 



The dyes employed for microscopical work, picric acid, 

 etc., can be used for dyeing the gelatine. The absorption 

 bands of a few suitable dyes are included in the diagram 

 of absorption spectra, page 159, but the screens made from 

 them will generally be found to give a longer trans- 

 mission band or pass other colours to a greater or less 

 extent, and the limits of the absorption bands are not 

 often clear-cut. On that account the screens are more or 

 less unsatisfactory, and the only way to determine their 

 suitability is to examine their transmissions with a 

 spectroscope or to make trial exposures. Their trans- 

 missions are in many cases very different from what their 

 colour suggests. Screens with sharply defined trans- 

 mission bands extending over a small part of the spec- 

 trum can be obtained by careful combination of screens, or 

 by incorporating two or more dyes in one screen, but as 

 such excellent sets are obtainable at small cost it is 

 seldom worth the trouble of making them. 



It is also impossible by inspection to judge of the 

 transmissions of ordinary coloured glasses, but, if suitable 

 pieces of orange, green, blue, or red glass can be obtained, 

 there is no more convenient colour screen to be had than 

 a circle which will fit the ring provided for stops beneath 

 the substage condenser. If used in that position it is 

 much more necessary that the glass be optically flat than 

 if placed at a distance from the condenser. 



Liquid Filters. Solutions are in certain respects better 

 even than glass. They pass more light, a larger range 

 of colours may be made, and they can be more easily 

 adapted to the subject in hand by addition of the colour- 

 ing matter or by dilution as required, and several very 

 useful solutions are available which cannot be used to dye 

 gelatine. In use they are unfortunately not quite so 

 easy to handle, and many workers on that account 



