192 HERMANN VON HELMHOLTZ 



your treatise on it. I therefore introduced some modifications 

 afterwards. In the After-images, as you will see, I have 

 modelled myself on you throughout. Contrast gave me the 

 most trouble; I have tried to clear up this chapter, but have 

 not yet got it right/ 



Part II of Physiological Optics deals with the Theory of 

 Visual Sensation, and treats in the first place of the various 

 forms of stimulation of the optic nerve, and then of its 

 excitation by light in particular, after which Helmholtz gives 

 a connected development of the theories previously published 

 by himself and others on simple and compound colours. In 

 connexion with the intensity and duration of visual sensation, 

 he gives a number of experimental methods and results, some 

 of which, e.g. the Psycho- Physical Law of Fechner, are sub- 

 stantially completed by some of his own later works, and lastly 

 he deals with the Theory of After-images and Contrast Pheno- 

 mena, on the lines indicated above, and illustrated by new and 

 interesting experiments. 



Two points may be selected from the wealth of new results 

 that had not previously appeared, either in his earlier works 

 or in the long series of profound theoretical deductions and 

 delicate experiments which combined his own with the work 

 of other investigators. 



For the purposes of physiological investigation it was neces- 

 sary to make a much more precise analysis of simple light than 

 was required by physical work in general, and in the first place 

 to investigate the theory of refraction in prisms, in so far as 

 this is essential to the production of pure spectra. While 

 formerly only the refraction of single rays of light in prisms, 

 not the position and character of the prismatic images, had been 

 determined, Helmholtz now investigated the prismatic images 

 formed by any kind of homogeneous light, when the eye looks 

 through a prism, or examines the light issuing from a prism 

 with lens or telescope, since these images must be regarded 

 as objects for the further optical images produced by the media 

 and lenses of the eye. 



If a ray is passed through different refractive media, and the 

 length of its path in each medium multiplied by the refractive 

 index of that medium, the sum of all these particular quan- 

 tities being termed the optical length of the ray, then the 



