Refracted and Diffracted Light. 351 



same distance from the screen, will be broader in propor- 

 tion as the refracting angle is greater. 



There is no certain standard, therefore, for refracted 

 light ; and, although its distinguishing qualities are ob- 

 servable only when it is in a state of considerable diver- 

 gence, all light which has lost the parallelism of its rays 

 by refraction must come under this denomination. 



Refracted light, then, includes the light of the sun as it 

 is transmitted by our atmosphere ; and, indeed, most of the 

 light with which we are acquainted. The light of the sun 

 is frequently so much refracted, particularly in the morn- 

 ing and evening, as to form distinct fringes, which may be 

 often observed upon the bars of windows, or upon the 

 edges of other objects placed to intercept it. 



I shall now proceed to shew the connexion between this 

 light and the diffracted light of Grimaldi and Newton. 



Diffracted light is formed either by admitting common 

 light through a small hole, or through a lens of very short 

 focus. The experiments made with light formed in these 

 different ways, render it probable that the effect produced 

 by the two methods of operating upon it, is nearly the 

 same. 



A lens differs from a prism, not only in its circular form, 

 which is calculated to refract the light of different colours 

 into concentric circles, but also in its having different 

 refracting powers at different distances from the centre ; in 

 consequence of which, the light that passes through it does 

 not open in the centre and form distinct concentric rings 

 of colours, but diffuses itself over the whole surface, form- 

 ing a circular image of white light, bounded by fringes of 

 the most refrangible colours. 



The light which has passed through a lens, then, is 

 refracted light ; but it is unequally refracted : it is so 

 refracted, that the whole of the light transmitted by the 

 lens converges to the same point, and, consequently, it 

 must be composed of a central ray, which, as it proceeds in 

 a right line, undergoes no refraction, surrounded by circles 

 of rays gradually becoming more highly refracted as they 

 recede from the centre. 



Light thus constituted will endeavour to form a distinct 

 circular spectrum with every circle of rays ; the whole of 



