7** Experiments for investigating 



order, and afterwards to give my sentiments upon the cause 

 of the formation of the concentric rings. The avowed in* 

 tricacy of the subject *, however, requires, in the first place, 

 a minute detail of experiments, and afterwards a very gra- 

 dual development of the consequences to be deduced from 

 them. 



As the word modification will frequently be used, it may 

 not be amiss to say, that when applied to light, it is intend- 

 ed to stand for a general expression of ail the changes that 

 are made in its colours, direction, or motion : thus, by the 

 modification of reflection, light is thrown back ; by that of 

 refraction, it is bent from its former course ; by the modi- 

 fication of dispersion, it is divided into colours, and so of 

 the rest. 



I. Cff different Methods to make one Set of concentric Rings 



visible. 



In the beginning of my experiments I followed the New» 

 Ionian example, and having laid the two object-glasses of 

 Huygens upon one another I soon perceived the concentric 

 rings. It is almost needless to say that I found all the New- 

 tonian observations of these rings completely verified ; but 

 as his experiments seemed to be too much confined for draw- 

 ing general conclusions, I endeavoured to extend them ; and 

 by way of rendering the methods I point out very clear, I 

 have given one easy particular instance of each, with the 

 addition of a generalization of it, as follows : 



First Method, On a table placed before a window I laid 

 down a slip of glass the sides of which were perfectly plain, 

 parallel, and highly polished. Upon this I laid a double 

 convex lens of 26 inches focal length, aiid found that this 

 arrangement gave me a set of beautiful concentric rings. 



I viewed them with a double convex eye lens of Sj inches 

 focus mounted upon an adjustable stand, by which simple 

 apparatus I could examine them with great ease ; and as it 

 was not material to my present purpose by what obliquity 

 of incidence of light I saw the rings, I received the rays 



♦ Newton'3 Optics, 4th ed, p. 288; endofObs. 12. 



Irom 



