182 HERSCHEL ON COLOURED RINGS. 



it may come up through the point of contact, and rcai h the 

 eye, having been transmitted through no more than three sur» 

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

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

 \\ad a hole of about half an inch in the middle. Over this 

 hole 1 laid a slip of glass with a 56-inch lens upon it; and 

 viewed a set ot 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 lens 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 having been 

 transmitted through the lowest surface of the subjacent glass. 

 The number of transmitting surfaces is therefore by this expe- 

 riment reduced to three; but I shall soon have an opportunity 

 of showing, that so many are not required for the purpose of 

 forming the rings. 



XXIV. Of the Action of the first Surface, 



Aetipn of the We have already shown, that two sets of rings may be seen 

 upper surface. ^ v us j n g a ] ens |4jd U p n a slip of glass ; in which case, there- 

 fore, whether we see the rings by reflection or by transmis^ 

 sion,nomore than four surfaces can be essential to their 

 formation. In the following experiments for investigating the 

 action of these surfaces I have preferred metalline reflection, 

 when glass was not required, that the apparatus might be 

 more simple. 

 This not af- Upon a plain metalline mirror I laid a double convex lens, 



fected by a having a strong emery scratch on its upper surface. When I 

 ' saw the rings through the scratch, they appeared to have a 

 black mark across them. By tilting the lens, I brought (be 

 centre of the rings upon the projection of the scratch, so that 

 the incident light was obliged to come through the sciatch to 

 the rings, and the black mark was again visible upon them, 

 but much stronger than before. In neither of the situations 

 were the rings disfigured. The stronger mark was owing to 

 the interception of the incident light, but when the rings had 



received 



