OTHER THEORIES 271 



forth the complementaries, which process once begun increases in 

 strength ; hence the loss of saturation of colours on prolonged fixation. 

 The primary, partial dissociation leaves a residuum of molecules which 

 undergo secondary dissociation, giving rise to the complementary 

 sensation. The residual molecules gradually dissociate spontaneously, 

 without adequate stimulation, and this process increases more and more 

 until there is equilibrium of direct and indirect dissociation, giving rise 

 to the neutral sensation. If the eye is then stimulated with white light, 

 the complementaries become more pronounced until, with the destruc- 

 tion of the residual molecules, equilibrium is re-established. 



Bonders thus introduced the ideas of chemical dissociation and 

 separation of the peripheral and central processes, both of which have 

 been further elaborated by later theorists. 



II. Ladd-Franklin's Theory 



Mrs Ladd-Franklin^ propounded the following theory. 



In the earliest stage of its development the visual sense con- 

 sisted only in the sensation of grey. The term " grey " is used to 

 express the whole series of black-white sensations. This sensation of 

 grey was brought about by the action upon the retinal nerve-endings 

 of a chemical substance set free by means of the decomposition of a 

 certain kind of molecule, the grey molecule. This molecule is composed 

 of an outer range of atoms, somewhat loosely attached to a firmer inner 

 core, and having various different periods of vibration. The decomposi- 

 tion of this molecule consists in the tearing off of its outer portion, which 

 then becomes the exciter of the nerve-endings and the immediate cause 

 of the sensation of grey. The tearing off is brought about by the ether 

 vibrations of the entire visible part of the spectrum, but in the greatest 

 amount by those near the middle part, as is shown by the sensations 

 of the totally colour-blind. 



In the course of the development of the colour-sense some of the 

 grey molecules become differentiated into colour-molecules in the 

 following manner. The atoms of the outer range segregate themselves 

 into three groups or pairs of groups at right angles to each other and 

 having three different average velocities. The adaptation between the 

 present structure of the retina (as regards colour) and the constitution 

 of physical light consists in the fact that the mean vibration periods of 



1 Ztsch.f. Psychol, u Physiol, d Sinnesorg. iv. 211, 1893; Mind, N. S. u. 473, 1893. 



