432 REPORT— 1886, 



of those fibres of the retina — as he calls them — which are calculated to 

 perceive red. Thus the theory of colour- vision, accepted mp to the present 

 date, was already established in principle. 



The knowledge of facts was too meagre as yet to prove the ideas of 

 Thomas Toung, and, therefore, they were gradually more and more dis- 

 regarded. It was not until thirty years ago that Maxwell and Helmholtz 

 saved them from ntter oblivion. The former even attempted an experi- 

 mental quantitative demonstration of the truth of these ideas. 



The methods and the results of Maxwell's investigations are too well 

 known to be specially dwelt upon here. But we must specially dwell 

 upon this point, that at the same time Helmholtz most emphatically de- 

 clared that colour-blindness (which in the meantime was better studied) 

 was the result of the absence of one fundamental sensation, although he 

 did not know Thomas Toung's ideas on this point. 



The results of Maxwell's investigation must be greatly valued, because 

 they contained the first measurements with spectral light ; but for the 

 very same reason, that they were the first, they could not be such that 

 final conclusions could be drawn from them. 



During the last ten years — that is, twenty years after Maxwell's in- 

 vestigations — the well-known scientists Kries, Frey, Bonders, and Lord 

 Rayleigh, supplied with greatly improved instruments, carried out more 

 exact measurements, which, however, extend only over certain parts of 

 the spectrum. These facts, and the circumstance that great facilities were 

 off"ered at the physical laboratory of the University in Berlin for investi- 

 gations of this kind, induced me to take up these measurements and to 

 extend them over the whole spectrum, and they were finally carried out 

 by me and my colleague Dr. Dieterici. 



§ 2. The investigation must begin with the reduction of the infinitely 

 large number of colour-sensations to the smallest possible number of 

 elementary sensations, which by their intensity and mutual relation produce 

 every possible kind of colour-sensation. This is a purely experimental 

 problem, whose solution can and will be made independent of every theo- 

 retical hypothesis. This is the reason why we choose the expression ' ele- 

 mentary sensation ' and not ' fundamental sensation,' because the latter 

 expression usually refers to a simple process going on at the terminal 

 of the optical nerve. This distinction is necessary, as will appear 

 later on. 



The first important simplification of our problem is afforded by the 

 fact that in the case of every individual we can produce every sensation 

 of colour by spectral light and their mixture. It seems expedient here to 

 lay down the following definition : given that the distribution of light in 

 our spectrum is such as we have it in a diff'raction-spectrum, then we 

 shall call ' curves of elementary sensation ' those curves which determine 

 the intensity of elementary sensation for any given wave-length. 



Having premised this definition we can at once proceed to a brief 

 description of the apparatus. 



It is an apparatus constructed by Prof. Helmholtz for mixture of 

 colours, and on this occasion improved by us in many details. It is a 

 spectroscope wiii an equilateral prism P (fig. 1), and two collimators C C ; 

 the telescope T paving instead of the eye-piece a slit S, at the focus of its 

 object-glass. Each collimator contains an achromatical iceland-spar J, 

 and in front of the slit a Nicol IT ; but for the present we shall disregard 

 these additions to the collimators. The slits 83 and S3 being illuminated 



