256 Prof. Grassmaiiu on the Theory of Compound Colours. 



a plane perpendicular to the axis of rotation; on the other 

 screen, and in the same plane, white light is suifered to fall in 

 any direction ; and in a direction perpendicular to the latter, 

 homogeneous light; the light being so selected tliat it may 

 have the same tint as the light to be examined. If now 

 this latter screen be turned upon the hinge, any desired ratio 

 of intensity may be given to the colourless and homogeneous 

 light which is dispersed on all sides from this screen. If the 

 first screen be then also turned upon the hinge, any degree 

 of intensity less than that produced by light falling on it per- 

 pendicularly may be given to the light diffused from it. In this 

 manner a position of the screens must necessarily be found, if 

 the light falling upon the second be not too strong, in which 

 both will produce the same impression upon an eye observing 

 them both at the same time. Such an apparatus would there- 

 fore be sufficient to determine mathematically all the elements 

 which may come under observation. 



The above statement, that the eye can only distinguish these 

 three elements directly, might indeed be doubted. And truly it 

 might be difficult to prove it directly, since there always remains 

 the possibility that one eye, by virtue of its peculiar organization, 

 might perhaps discover diff'ei'ences invisible to another. How- 

 ever, for our purpose the fact is perfectly sufficient, that hitherto 

 no observer has been able to mention another element determi- 

 ning the impression of colour ; and moreover, in ordinary lan- 

 guage, we only recognise these thx-ee elements in the description 

 of this phsenomenon, so that we may aflSrm with certainty that 

 hitherto only these three elements have been observed in the 

 impression of colour ; in the following arguments we shall only 

 go back to this assertion. 



In the second place we assume, " that if one of two mingling 

 lights be continuously altered (whilst the other remains un- 

 changed), the impression of the mixed light also is continuously 

 changed.^' 



We say that an impression of light is continuously altered 

 when the two intensities (that of the colour and that of the in- 

 termixed colourless light) are continuously changed ; and where 

 tlie tint also, svipposing that the intensity is not zero, continuously 

 changes. If the colour have no intensity, the light is colourless ; 

 and consequently by the continuous diminution of the intensity 

 to zero, a tint may gradually pass over into another completely 

 separated from it, if the intensity of the latter increase conti- 

 nuously from nothing upwards. It scarcely needs to be men- 

 tioned, that the case in which one or more of the elements deter- 

 mining the impression remain the same must be regarded as 

 coming within the idea of continuity. Now as regards the con- 



