iSs Dr. Herschel’s Experiments for investigating 
cation of reflection, light is thrown back ; by that of refraction, 
it is bent from its former course ; by the modification of dis- 
persion, it is divided into colours, and so of the rest. 
I. Of different Methods to make one set of concentric Rings 
visible. 
In the beginning of my experiments I followed the New- 
tonian 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 drawing 
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 2 6 inches focal length, and found that this arrangement 
gave me a set of beautiful concentric rings. 
I viewed them with a double convex eye lens of 2-§- inches 
focus mounted upon an adjustable stand, by which simple ap- 
paratus I could examine them with great ease ; and as it was 
not material to my present purpose by what obliquity of inci- 
dence of light I saw the rings, I received the rays from, the 
window most conveniently when they fell upon the lens in an 
angle of about 30 degrees from the perpendicular, the eye 
being placed on the opposite side at an equal angle of elevation 
to receive the reflected rays. 
