92 COLOUR VISION 



white stimulation appears to have a greater effect than that of the 

 preceding white stimulation, though the authors can give no explanation 

 of the fact. If a colour stimulation follows instead of white, the effect 

 on the action-time depends largely upon the similarity or dissimilarity 

 between this light and that which is being tested. 



THE TALBOT-PLATEAU LAW. 



When periodic excitations follow each other with sufficient rapidity 

 the resulting sensation is one of continuous light of uniform brightness. 

 Talbot 1 and Plateau 2 investigated the relationship of the brightness 

 of the individual periodic stimuli and of the resultant sensation. Their 

 conclusions are usually known as the Talbot -Plateau Law, which states 

 that the resultant impression is the mean of the periodic impressions, 

 i.e., the resultant brightness is that which would have arisen if the 

 amount of light intermittently reaching the retina had been uniformly 

 distributed over the whole period of stimulation, v. Helmholtz confirmed 

 the law for ordinary physical intensities of light. Fick 3 found some 

 deviations, low intensities giving a continuous sensation brighter than 

 the intermittent. 0. C4riinbaum 4 also found deviations for high in- 

 tensities, the intermittent light being brighter than the continuous. 



The accuracy or otherwise of the Talbot-Plateau Law is of consider- 

 able importance in the investigation of colour vision, for the principle 

 of the episcotister depends upon its accuracy. Abney finds that the 

 adjustable sectors, i.e., those which can be alter.ed during rotation, are 

 only available for accurate measurement when the angles of aperture 

 lie between 180 and 10, chiefly owing to the errors in reading being 

 proportional to the angles of aperture. With fixed sectors angles of 

 2 or even 1 can be used. It is not certain, however, that the method 

 is reliable for very low intensities of light, and for these the annulus 

 (v. p. 5) is to be preferred. The experiments of Abney, Lummer and 

 Brodhun 5 , Hyde 6 and others show that the law applies accurately over 

 a wide range of physical intensities. Hyde's experiments were very 

 carefully conducted, the probable errors of measurement being under 

 O'l per cent. The average deviation of the observations for any given 

 angular aperture of the sectors was in no case as large as 0'2 per cent. 



1 Lond. r FAin. and Dublin Phil. Mag. V. 327, 1834. 



2 Ann. d. Phys. u. Chem. xxxv. 457, 1835. 



3 Arch.f. Anat. Phyxiol. u. wiss. Med. 754, 1SG.3. 



4 J. ofPhysiol. xxn. 433. 1898. 5 Ztsch.f. Inxtrumentenkunde. xvi. 299. 1896. 

 6 Bull, of the Bureau of Standards, Washington, n. ], 1906. 



