538 Intelligence and Miscellaneous Articles. 



will be varied by employing different metals, different gases, and 

 working under different pressures ; I shall have the honour of 

 making known to the Academy the results. — Comptes Bendus de 

 lAcademie des Sciences, May 8, 1882, t. xciv. pp. 1271-1273. 



ON THE EMPLOYMENT OF ROTATING DISKS FOR THE STUDY OF 

 COLOURED SENSATIONS. THE RELATIVE INTENSITY OF 

 COLOURS. BY A. ROSENSTIEHL. 



In continuing my researches upon the coloured sensations, I 

 have been led to examine a problem which has hitherto appeared 

 inaccessible — that of the relative intensity of colours. What 

 common measure can there be between a red and a yellow, a blue 

 and a violet? It seems at first sight impossible to set up a 

 numerical comparison between values which appear so different in 

 kind ; and yet this comparison is possible, thanks to a well-known 

 special property of the eye. 



Between the different colours which have received names an 

 infinity can be intercalated which form the insensible transition 

 from the one to the other without a break. The series is 

 continuous, and closes upon itself like the perimeter of a polygon. 

 Besides (and I cannot sufficiently insist upon the peculiarity), this 

 whole includes an infinity of sensations which, associated in twos, 

 produce a sensation identical for all, that of white. We find 

 nothing analogous in the sensations of the ear ; and it is this 

 highly characteristic circumstance that gives a common measure 

 between the different colours. 



In my previous Notes, adopting and precisely defining Young's 

 theory, I laid it down as a principle that the sensation of white 

 results from the equal excitation of three primary sensations. 

 Consequently every pair of complementary colours represents in 

 its combination the mixture of the three primary colours at equal 

 intensity. This notion of equal intensity was introduced into a 

 graphic construction, which was discussed; and I verified the 

 principles on which it is based. 



It is necessary to distinguish between intensity of coloration 

 and total luminous intensity. By adding the sensation of white 

 in different proportions to a coloured sensation of constant inten- 

 sity, colours of the same intensity of coloration, but of different 

 total luminous intensity, are obtained. These colours have for 

 their common measure the angle of the sector of the comple- 

 mentary necessary to extinguish all coloured sensation ; and, 

 generally, the angle of the sector of the complementary colour is in 

 the inverse ratio of its coloration-intensity. The angle of the white 

 sector * is in the direct ratio of the total luminous intensity. 



* It will be remembered that, in all my experiments, the intensity of the 

 sensation of white is measured by the angle of a sector painted white with 

 sulphate of barium, which is set in rapid rotation in front of an absolutely 

 black orifice. 



