THE PHYSICAL BASIS 5 



can be accurately focussed upon a screen. Such a projected spectrum 

 can then be viewed by several observers at the same time, a very con- 

 siderable advantage in testing colour vision. The use of a projected 

 spectrum necessitates further care in detail, for the character of the 

 spectrum will depend upon the optical properties of the screen 1 . A 

 matt white surface must be used and that obtained with magnesium 

 oxide is probably best. 



In order to obtain the most accurate information from the experi- 

 ments the observations must be as far as possible quantitative and not 

 merely qualitative. In many physiological experiments this counsel of 

 perfection cannot be complied with and we are reduced to the informa- 

 tion which can be obtained from merely qualitative observations. When, 

 however, it is possible to obtain quantitative results it is generally 

 necessary to have a constant light for purposes of comparison. Now, 

 photometry is admittedly one of the most faulty of physical measure- 

 ments, chiefly, to use a paradox, because it is, in most cases, not really 

 physical but physiological. One of the most important and unique 

 features of Sir William Abney's apparatus is that the intensity of the 

 comparison light bears a constant physical relationship to that of 

 the spectrum used, since it is obtained by the reflection of a portion 

 of the original beam of light from the surface of the first prism. Hence 

 any variation in the original beam will cause similar and simultaneous 

 variations in both the spectrum and the comparison light 2 . Measurable 

 changes in the intensity of the light are best obtained by the use of rotating 

 sectors, sometimes called the episcotister (Aubert), or by the annulus, 

 a gelatine wedge impregnated with ivory black 3 . Reduction of intensity 

 by means of Nicol prisms as in v. Helmholtz' spectrophotometer, may 

 not be free from error, since quite an appreciable amount of polarisa- 

 tion of the light is produced by the prisms used to form the spectrum. 

 Much of the German work has been done by this method, and care has 

 by no means always been taken to calculate the corrections necessary 

 owing to this cause. 



Pure spectral colours rarely occur in nature, and much of the litera- 

 ture on colour vision is devoted to observations with pigments, coloured 

 glasses and so on. It is necessary, therefore, to say a few words about 

 these complex colours, chiefly with the object of putting the reader 

 upon his guard. When white light passes through a red glass or trans- 

 parent red fluid certain rays are absorbed. The red rays are transmitted 



1 Abney, p. 46. 2 Ibid. Chap. IV. 3 Ibid. Chap. vi. 



