SCKEENS PLATES EXPOSUKE 218 



dyes. The strength of solution of these dyes may conveniently 

 be in a proportion of one in one thousand, and the width of the 

 cell in which they are contained should not in this case be less 

 than five millimetres. The absorption-spectra of all aniline 

 dyes vary very largely with the strength of solution ; or, what 

 is much the same in effect, the width of cell in which they are 

 contained. A green dye which transmits a specially narrow 

 band, between wave-lengths 4500 and 5000, is Victoria green, 

 and a solution of the strength mentioned makes a very effi- 

 cient filter. Tartrazine and acid-green, in separate solutions, 

 make an efficient and nearly monochromatic-green screen 

 (Fig.67/). 



For blue filters there is not such a large demand as there is 

 for green or yellow. The most useful chemical solution is 

 ammoniosulphate of copper, and this efficiently cuts off the red 

 end of the spectrum. Most blue aniline dyes transmit a good 

 deal of red, hence they are not generally used except in con- 

 junction with a screen that will cut off the red portion. A 

 solution of night-blue, one in one thousand, transmits a fairly 

 narrow band between wave-lengths 4000 and 5000, and may be 

 found useful under suitable conditions. Messrs. Zeiss have 

 recently introduced a blue glass which very efficiently absorbs 

 the whole of the red end of the spectrum ; in order to obtain 

 the best result, it should be ordered for and adapted by 

 them to the source of light with which it is to be used. 



Methylene-blue transmits a definite band in the red, and 

 must be used in conjunction with a piece of signal-green 

 glass, or some similar absorptive substance which does not 

 allow red to pass. 



It is obvious that in selecting colour-screens due regard must 

 be had to the source of light in use. Very weak sources, such 

 as an oil lamp, would only require weak solutions and thin 

 layers to effect the necessary absorption, whereas with an 

 illuminant of high intensity, such as an electric arc, or lime-light, 

 a much stronger solution and a thicker layer must be used. 



It is almost impossible to give any absolute directions as 

 to the strength of the solutions in view of these variable 

 factors, and also owing to the fact that aniline dyes, even 

 when described by the same names, are themselves variable. 

 A test should always be made with a small direct-vision 



