1] APPLICATION AND MEASUREMENT OF LIGHT 157 



A second method of getting monochromatic light is by the 

 use of flames tinged with various volatilized metals. Of these, 

 lithium salts give reds at A, = 0.67^, and X = 0.61/i, sodium salts 

 give a pure yellow light of A, = 0.59//,, thallium salts (poisonous 

 vapor) a green at about \ = 0.54/u,, and indium salts a blue and 

 a violet, both beyond \ = 0.46/i. The number of metallic 

 vapors which give even nearly monochromatic light is, however, 

 not large. 



A third method for producing monochromatic light is found 

 in the use of solutions of pigments. Such solutions may be 

 held in deep glass vessels whose back and front glass surfaces 

 are parallel and near together. In the following table are given 

 in the first column the names which may be applied to differ- 

 ent parts of the spectrum, following HELMHOLTZ (Handbuch, 

 p. 251); in the second column the pigments which while dry 

 give corresponding spectral colors in diffuse daylight and which 

 may also be used in making solutions ; and in the third column 

 certain pigments which in solution YUNG ('78, p. 251) found to 

 transmit almost exclusively the part of the spectrum named in 



the first column. 



TABLE XVII 



Solutions made up from these pigments should, however, be 

 examined spectroscopically before using to make sure of the 

 purity of the color. 



* Also a solution of iodine in carbon disulphide. (PRINGSHEIM, '80, p. 409.) 

 t F to H is given by ammoniated copper sulphate, CuSo4-4NH 3 + H 2 

 (PRINGSHEIM). 



