LECTURE-ILLUSTRATION OF COMPLEMENTARY COLOURS. 

 BY NIK. VON KLOBUKOW. 



The method consists in dissolving the pigment-colours in suit- 

 able proportions in solvents which, differing considerably in specific 

 gravity, are not soluble in each other, and neither of which dis- 

 solves the body in the other, and then, by violently shaking the 

 solutions, and thus as it were effecting a mixture of the physical 

 molecules, to bring about a mixture of colours. 



The perception of colours is here brought about by direct 

 action, as in the experiment of mixing colours by reflexion, and 

 not, as in the colour-disk, by after-action of the luminous im- 

 pression. 



Owing to the above-mentioned properties of the solutions, the 



Intelligence and Miscellaneous Articles. 3i) ( J 



one turn in a second, after having fixed on the periphery of this 

 disk a very small white sector of 1° or 2°, and a height of 5 to 10 

 millim. In this way two necessary conditions are realized — a 

 motion so rapid that there is a persistent image of this sector as 

 extended as possible, and at the same time excitations at such 

 intervals that the persistent images do not run into each other. 

 If, then, the view is fixed rigidly towards a point at which the 

 sector passes, which is the delicate and essential condition of the ex- 

 periment, the persistent annular image of the object is seen to be 

 channelled, and presents a certain number of dark zones regularly 

 spaced out on the light ground. An intense light is not needed 

 for this experiment. 



The extent of the successive zones on the retina as w T ell as their 

 frequence is easily calculated. 



It is found that the apparent interval between two dark zones 

 on the disk diminishes with the distance from the eye. The image 

 on the retina of this interval, on the contrary, remains constant. 



For the same distance of the disk from the eye the interval in 

 question varies with the velocity of the former, and, what is a 

 point of capital importance, inversely as this velocity. This is not, 

 then, a case of direct oscillations due to excitation, for they are, on 

 the contrary, spaced out in proportion to the velocity of the disk. 



This fact can only be explained by assuming that the object in 

 moving over the retina is, in relation to the induced oscillation, in 

 analogous conditions to those of an observer who moves away from 

 a source of sound. If the retinal undulation which we have 

 actually observed in the experiment with the black band travels 

 with a constant velocity over the retina, the passage of a luminous 

 image moving with a suitable rapidity should find this membrane 

 in conditions periodically varying, in which the perception of the 

 object will be alternately favoured or opposed. The distance of 

 two neighbouring maxima and minima, which represents the appa- 

 rent wave-length of this retinal undulation, should obey the relation 

 expressed by Doppler's formula. This is what in fact is confirmed 

 by experiment. — Comjptes Rendus, July 20, 1891. 



