40 DISPERSION. 



by the changes which that light undergoes in refractions or 

 reflexions. 



(49) In the experiments hitherto described, the analysis 

 of solar light, or its resolution into its simple components, is 

 far from being complete, inasmuch as there is a considerable 

 mixture of the different species of simple light in the spectrum. 

 This will be evident, if we consider that, as the several pencils 

 of homogeneous light suffer no dilatation by the prism, each will 

 depict on the screen a circular image, equal in magnitude to the 

 unrefracted image of the Sun ; and accordingly the spectrum 

 will consist of innumerable circles of homogeneous light, 

 whose centres are disposed along the same right line, and whose 

 common diameter is that of the Sun's unrefracted image. 

 Wherefore the number of such circles mixed together in the 

 spectrum, is to the corresponding number in the unrefracted 

 image of the Sun, as the interval between the centres of two 

 contingent circles, or the breadth of the spectrum, to the in- 

 terval between the centres of the extreme circles, which is the 

 length of the rectilinear sides. The mixture in the spectrum, 

 therefore, varies as the breadth of the spectrum divided by 

 its length ; and if the breadth can be diminished, the length 

 remaining the same, the mixture will be diminished in pro- 

 portion. 



Newton's method of diminishing the breadth of the spec- 

 trum, or the diameter of the Sun's unrefracted image, was as 

 follows. The solar beam, admitted through a small circular 

 aperture, is received upon a lens of long focus, at the distance 

 of double its focal length from the aperture ; and at the same 

 distance beyond the lens will be formed a distinct image of 



lower refrangibility, when excited by those of a higher. This property belongs 

 to the solution of sulphate of quinine, and to certain coloured glasses. Professor 

 Stokes has denominated it fluorescence. 



