40 



Doctor Tenney, on PrismaUck Colours. 



be proved, that all rays of the same colour possess precise 



ly the same deg 



of refrangibility 



Two circumstances 



concur to render it, at least probable that they do not. 

 The first is, that the space occupied by any particular col- 

 our, in the prismatick picture, is so 



perture, through 



i 



so much wider than the 

 hich the light is admitted upon the 

 prism, for the experiment. For, if all the rays of that colour 

 were equally refrangible, the picture could be no wider than 

 the aperture ; and would be terminated by well defined 

 edges : which is the result of this law of refractions, that par- 

 allel rays, of the same refrangibility, perpetually maintain 

 their parallelism. The second circumstance is, the unequal 

 widths of the several colours in the prismatick picture. Now, 



as this cannot arise from the 

 rays, which would produce on 



g 



or less quantity of 



different deg 



of in ten 



sity, it must proceed from this: that the rays of those col- 

 ours which form the widest pictures, possess different degrees 

 of refraiigibihty among themselves : in consequence of which, 

 they are scattered over a greater space. This being allowed, 

 it is highly probable, that some red, and some yellow rays 

 may be equally refrangible; in which case, they must, at 

 their exit from the prism, be necessarily blended. The same 

 may hold good of the other rays, some yellow and blue, 

 some blue and violet remaining unseparated ; from all which 

 combinations will arise the orange, green, and purple colours. 

 Should it be here asserted, that the'se arguments prove on^ 

 ly that rays of the s{ 



imperfection of the 



suffer 



tiiat although 



refracting 



it would be 



a dispersion from the 

 medium, I would answer, 

 possible to account for thQ 



second 



■• 



