214 Different Refrangilility [Book III. 



retina is fmaller than it would have been, had thefc 

 rays not been intercepted in their natural progrefs ; 

 and by the divergence of the rays the object is repre- 

 fented with lefs clearnefs than it would otherwife have 

 had, fince from this caufe a lefs quantity of light in 

 fact enters the pupil of the eye. All concave lenfes 

 have a negative or virtual focus, which is a point cor- 

 refponding with the divergence of parallel rays inci- 

 dent on the furface of the lens. 



Light is, however, not fo fimple a fubftance as it 

 may be fuppofed upon fuperficially confidering its 

 general effects ; it is indeed found to confift of particles 

 which are DIFFERENTLY REFRANGIBLE, that is, fomc 

 of them may be refracted more than others in palling 

 through certain mediums, whence they are fuppofed 

 by philofophers to be different in fize. The common 

 optical inilrument, called a prifm, is a triangular piece 

 of glafs, through which, if a pencil or collection of 

 rays is made to pafs, it is found that the rays do not 

 proceed parallel to each other on their emergence, 

 but produce on an oppofite wall, or any plain furface 

 that receives them, an oblong fpectrum, which is va- 

 rioufly coloured, and it confequemly follows that fome 

 of the rays or particles are more refrangible than 

 Others. 



The fpectrum thus formed is, perhaps, the moft 

 beautiful object which any of the experiments of phi- 

 lofcphy prcfent to our view. The lower part, which 

 confifts of the leaft refrangible rays, is of a lively red, 

 which, higher up, by infenfible gradations, becomes an 

 orange ; the orange, in the fame manner, is fucceed- 

 cd by a yellow -, the yellow, by a green j the green, 

 by a blue ; r.fter which follows a deep blue or indigo ; 

 and Jaftiy, a faint violet. 



In the two fucceeding chapters the principles which 



have 



