THE LUMINOSITY OF THE SPECTRUM 45 



and Kennelly, Fleming and others^. This method, however, is too 

 inaccurate for the purpose in view, though Crova's curves^ are in- 

 teresting^. 



Charpentier^ and Haycraft^ used the method of finding the dis- 

 tance at which small areas of different colours become invisible. Another 

 method, based on the fact that the size of the pupil depends, inter alia, 

 upon the intensity of the light, has been advocated by Gorham^, and 

 has also given good results with coloured papers. Rivers', Martins^, 

 and Briickner^ have suggested other methods. 



Heterochromic photometry has become of importance technically 

 since about 1880 on the introduction of the arc light for commercial 

 purposes. Hence a considerable amount of attention has been devoted 

 to it^°. Apart from the equality of brightness method adopted by Abney 

 and Festing, and Konig and Ritter, the method which provides most 

 accurate data for physiological purposes is flicker photometry, suggested 

 by Talbot (1834), but first used by Schafhautl (1855), and subsequently 

 by Rood^^, Polimanti^^ and others. When before two lights of different 

 luminosities an episcotister (a metal disc containing alternate closed 

 and open sectors) is rapidly rotated, an unpleasant flickering of each 

 light is seen at certain rates of rotation. Now it is found that the 

 flickering ceases at the same speed of rotation of the episcotister for the 

 two lights when their luminosities are the same. The causes of flicker 

 are complex and will require further attention later (Section V, Chap. i). 

 The results obtained are not free from criticism, but if due precautions 

 are taken they agree well with those yielded by the equality of brightness 

 method. 



Konig's luminosity curves for the gas spectrum with different light 

 intensities are given in Fig. 10. They were obtained by the equality 

 of brightness method. 



We are concerned here only with the higher intensities {F, G, H). 

 The curves have the same ordinate at 535 /x/z, because that ordinate 

 was taken as unity. It will be noted, however, that they all cross 

 and reverse their relative positions. This is due to the Purkinje 



1 Trotter, Illumination, London, 1911. 



■^ Ann. de Chim. et de Phys. vi. 528, 1885 : La Lumiere elec. xvm. 549, 1885. 



* Trotter's Illumination, p. 171. * La Lumiere et les Couleurs, Paris, 1888. 



* J. of Physiol. XXI. 126, 1897. « Proc. Roy. Soc. Lond. xxxvn 425, 1884. 



' J. of Physiol, xxn. 137, 1897. ^ Beitrdge zur Psychol, u. Philos. i. 95, 1896. 



* Arch. f. d. ges. Physiol, xcviii. 90, 1904. lo Trotter, he. cit. 101. 

 " Amer. J. of 8c. xlvi. 173, 189:5 ; Science, vu. 757 ; viii. 11, 1898. 



12 Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xix. 263, 1899. 



