62 APPLICATION OF FIELD'S CHROMOMETER. 



3rdly. By the application of the convex lenticular prism and 

 chromascope to the black spot on the white ground, the same 

 three original coloured rays are rendered distinctly evident and 

 homogeneous, showing that the prism has the power to decom- 

 pose the light which has been exhibited in the form of shade, 

 and that the same identical coloured rays are, by transverse 

 interposition, rendered anti-radiants, and consequently inca- 

 pable of reflection. But the action of the concave prism 

 immediately restores the rays so refracted to the original state 

 of shade or black spots ; and that the same effects are produced 

 by the alternate application of the convex and concave prisms 

 respectively, on the white spot, on a black or dark ground. It 

 therefore appears fully established by the experiments of Mr. 

 Field, and by those of Sir David Brewster, that there are but 

 three original coloured rays in the composition of light, as 

 exhibited by the solar beam. 



The black lines which striate the spectrum produced by the 

 common triangular prism, as described by Wollaston and Fraun- 

 hoffer, are not to be found in their confused state in the an- 

 nular iris produced by the prism of Field, but are visible in 

 one continuous circle always adjoining the blue, without inter- 

 ference or intersection of any other coloured ray in the aureola. 



The three original coloured rays can therefore appear sepa- 

 rate, distinct, and homogeneous and can form conjointly 

 white radiant and reflected light, or dark shade, capable of 

 refraction, but not of reflection. 



The analogy is worthy of notice, that oxygen, nitrogen, and 

 hydrogen are distinct and homogeneous ; that they form (as 

 here presumed) carbon, which is black and incapable of reflec- 

 tion, but in its crystallized state, the diamond, is translucent, 

 and possesses the power of refraction and reflection. 



To adjudicate the proportionate quantum of space actually 

 occupied by each coloured ray in the formation of a given area 

 of light, Mr. Field adopted his very ingenious instrument, 

 already described, and exhibited in the several Figures, 5, 6, 7, 



