188 HERSCHKL 0NT COLOURED RINGS. 



third surface has a power of forming distorted rings, and that 

 consequently a reflection from one that is perfect must have 

 a power of forming rings without distortion, when it is com- 

 bined with a proper second surface, 

 both of the pri< When the defective slip e# glass, with a perfect lens upon 

 condary sets" **> *** P^ acec ^ upon a metal line mirror, I saw the secondary 

 set affected by distortions of the rings that were perfectly 

 like those in the primary set ; which proves, that a polished 

 defect in the third surface will give modifications to the 

 rays that form the rings by transmission as well as by reflec- 

 tion. 



XXVII. The Colour of the reflecting and transmitting Swr- 

 faces is of no consequence. 



The colour of I laid seven 54-inch double convex lenses upon seven eo- 



the surfaces of i ourec l pj ece s of plain glass. The colours of the glasses 

 no conse- f . ^ ? . n . 



fjuence. were those which are given by a prism, namely, violet, in- 



digo, blue, green, yellow, orange, aud red. The rings re- 

 flected from each of these glasses were in every respect 

 alike ; at least so far that I could have a black, a white, a 

 red, an orange, a yellow, a green, or a blue centre with 

 every one of them, according to the degree of pressure I 

 used. The lenses being very transparent, it may be admit- 

 ted, that the colours of the glasses seen through them would 

 in some degree mix with the colours of the rings; but the 

 action of the cause that gives the rings was not in the least 

 affected by that circumstance. 

 "* I saw the rings also by direct transmission through all the 

 coloured glasses except a dark red, which stopped so much 

 light, that I could not perceive them. The colour of the 

 glasses, in this way, coining directly to the eye, gave a strong 

 tinge to the centres of the rings, so that instead of a pure 

 white I had a blueish white, a greenish white, and so of the 

 rest ; but the form of the rings was no less perfect on that 

 account. 



X X V 1 1 1. Of the Action of the fourth Surface. 



of rhe We have already seen, that a set of rings may be com- 



4th surface, pjttely formed by reflection from a third surface, without 



the introduction of a fourth; this, at all events, must prove, 



that 



