XXII 



optics were closely intertwined. In his collected works twenty-four 

 papers are included under these heads, but most of his conclusions 

 are published in his ' Handbuch der Physiologischen Optik.' The 

 first edition of this great work was completed in 1866. Another was 

 passing through the press at the time of the author's death. 



Adhering approximately to the chronological order of his inquiries 

 into these subjects, the two first were devoted to the ophthalmoscope 

 and to colour mixtures respectively. 



The problem of how to see the interior of the eye did not involve 

 any novel or recondite principles, but was practically extremely diffi- 

 cult. A hint as to the method of solution was obtained from the 

 recorded fact that an observer, wearing spectacles, noticed that a 

 friend's eye appeared to him to glitter when light was reflected into 

 it from his own glasses. The account which Helmholtz gave of the 

 invention of the instrument, to the construction of which this 

 afforded the clue, is a model of the combination of mathematics and 

 experiment to attain a practical result. 



A modification of the original ophthalmoscope devised by Th. 

 Ruete, in which the observer looks through a hole in the middle of a 

 mirror, is that now used by oculists. Helmholtz published a paper 

 on this form of the instrument, but though he regarded it as the best 

 for medical purposes, he preferred the earlier type for research. 



On the theory of mixed colours and the closely related problem of 

 colour blindness, Helmholtz does not seem to have held with perti- 

 nacity the views generally attributed to him, to which, no doubt, he 

 at one time inclined. 



In his earliest experiments he combined the spectra produced by 

 two slits inclined in opposite directions at 45° to the vertical. He 

 concluded that at least five colours, viz., red, yellow, green, blue, and 

 violet, were necessary if all the colours of the spectrum were to be 

 matched by compound tints, but that if three only were to be 

 selected the best choice was that of Thomas Young, viz., red, green, 

 and violet. He also explained the reasons why the tints resulting 

 from a mixture of coloured powders are different from those pro- 

 duced by mingling rays of homogeneous light. Later and more 

 refined experiments led him to adopt unreservedly the theory of three 

 fundamental colours. 



He rejected Brewster's view that three kinds of light — red, green, 

 and blue, co-exist in each homogeneous ray. The explanation of 

 compound colours as due to the mixture of three fundamental tints 

 could only be understood as applied to subjective phenomena not as 

 describing external physical facts. It is the reduction of colour sensa- 

 tions to a mixture of three fundamental sensations, not the analysis 

 of light waves into three fundamentally different types of vibration. 



" In this sense," he said, " Thomas Young solved the problem cor- 



