the Cause of coloured concentric Rings, IS9 



XXITT. Of tlie transmitting Surfaces. 



.Jt would seem to be almost self-evident, that when a set 

 of rings is seen by transmission, the light which occasions 

 them must come through all the four surfaces of the two 

 glasses which are employed ; and yet it may be shown that 

 this is not necessary. We may, for instance, convey light 

 ii^io the body of the subjacent glass through, its first surface, 

 and let it be reflected within the glass at a proper angle, so 

 that it may come up through the point of contact, and reach 

 the eye, having been transmitted through no more than three 

 surfaces. To prove this I used a small box, blackened on 

 the inside, and covered with a piece of black pasteboard, 

 which had a hole of about half an inch in the middle. Over 

 this hole I laid a slip of glass with a 56-iuch lens upon it ; 

 and viewed a set of rings given by this arrangement very 

 obliquely, that the reflection from the slip of glass might be 

 copious. Then guarding the point of contact between the 

 kns and the slip of glass from the direct incident light, I saw 

 the rings, after the colour of their centre had been changed, 

 by means of an internal reflection from the lowest surface of 

 the slip of glass ; by which it rose up through the point of 

 contact, and formed the primary set of rings, without ha- 

 ving been transmitted through the lowest surface of the sub- 

 jacent glass. The number of transmitted surfaces is there- 

 fore by this experiment reduced to three; but I shall soon 

 have an opportunity of showing that so many are not re- 

 quired for the purpose of forming the rings. 



XXIV. Of the Action of the first Surface, 



We have already shown that two sets of rings maybe seen 

 by using a lens laid upon a slip of glass ; in which case, 

 therefore, whether we see the rings by reflection or by trans- 

 mission, no more than four surfaces can be essential to their 

 formation. In the following experiments for investigatinor 

 the action of these surfaces I have preferred metalline reflec- 

 tion, when glass was not required, that the apparatus might 

 be more simple. 



Upon a plain metalline mirror I laid a doubk convex lens, 



having 



