4 COLOUR VISION 



o . o 



table gives the principal lines in Angstrom units (1 A.U. one ten 

 millionth part of a millimetre = O'l /j.^). 



The most convenient method of calibration, however, is by the 

 mercury lines as given by the " mercury arc 1 ." 



In spite of the necessity for calibration the prismatic spectrum is 

 more generally suited than the diffraction spectrum for physiological 

 experiments on account of its greater brightness and relative freedom 

 from scattered light. 



Whatever spectrum be employed the source of light must be constant. 

 Lights which we commonly regard as giving " white light," such as 

 sunlight, the arc light, incandescent light, and so on, vary much in 

 character and consequently in the constitution of their spectra. 

 Sunlight varies so much that it is generally unsuitable for the purposes 

 in view, the variations being not only in intensity but also in com- 

 position, owing to the unequal absorption of different rays by the 

 atmosphere, and this absorption again varies greatly according to the 

 amount and nature of the matter suspended in the air. The arc light 

 is the most satisfactory, and after this probably the Nernst lamp, though 

 the latter has not yet been sufficiently investigated 2 . Less satisfactory 

 are gas light, petroleum and so on, but as many of the experiments of 

 earlier observers have been made with such sources they have to be 

 considered if these researches are to receive due appreciation. Some 

 sources of illumination, especially used for investigation of the ultra- 

 violet rays, such as the Schott uviol mercury vapour lamps, are wholly 

 unsuited, since they do not give continuous spectra. For experiments 

 on colour vision many such details which cannot be discussed here must 

 be attended to 3 . 



Suffice it to say that by taking proper precautions it is possible to 

 obtain a spectrum which is practically constant during the time necessary 

 to take a series of observations and which can be reproduced from time to 

 time with a minimum of variation. 



If such a spectrum is viewed through the eyepiece of an ordinary 

 spectroscope a direct spectrum is seen. This method has usually been 

 adopted, as for example by Aubert, von Helmholtz, Clerk-Maxwell and 

 others. By a slight change in the optical arrangements the spectrum 



1 Watson, Practical Physics, p. 309, 1906. 



2 Abney, Researches in Colour Vision, 1913, Chap. v. ; Golant, Ztsch. f. Sinncsphysiol. 

 XLIII. 70, 1908. 



3 E.g. Tigerstedt, Handb. </. p/iysiol. Methodik, Bd. in. Abt. 2, Sinnesphysiologie n. 

 Leipzig, 1909. 



