460 M. E. Wilde on the Untenableness of 



not yet found a passage, the bluish bow MN is observed, which 

 Newton was the first to notice and explain. In the portion 

 of the base between MN and AD, where all the rays are re- 

 fracted, the reflected light is for this reason very dull. 



Having laid the basis DB of the prism upon the summit of 

 a very flat convex lens, I pressed both glasses forcibly toge- 

 ther, and inclined the eye towards the base, so that I could 

 see the place where the glasses touched between q and AD. 

 Here were exhibited a considerable number of rings, such as 

 those observed in daylight with two very flat convex lenses ; 

 a complete central spot of deep black was visible at the same 

 time. The eye being bent still lower while the glasses re- 

 mained unaltered, I observed the place of contact between q 

 and p ; the central spot appeared no longer black, but a most 

 vivid green. When the eye was inclined still more, colours 

 of the brightest yellow, orange, and at last red, were exhibited 

 successively, until finally, when the eye was sunk still lower, 

 so that the place of contact came into the vicinity of total re- 

 flexion, the deep black spot again appeared complete as 

 before. 



This experiment is decisive. It proves that the hue of the 

 central spot depends upon transmitted light, and for ever sets 

 aside the possibility of the assumption, that the origin of the 

 spot is to be sought in interference. The colours thus obtained 

 are perfectly homogeneous ; and hence, if the idea of inter- 

 ference could in the least be admitted, the central spot must 

 remain invariably black. This however is not the case. The 

 colours green, yellow, orange, which the central spot assumes, 

 are as vivid and pure as those obtained from the best flint- 

 glass prisms, which is especially evident when the glasses are 

 laid upon black paper and the light prevented from falling 

 upon the lens itself. Of this I convinced myself by placing an 

 opake screen on that portion of the lens turned towards the 

 light which was uncovered by the prism, and thus holding 

 back those rays which could fall upon the base from below. 

 Had the lens a stronger convexity, the colours of the central 

 spot, as may be readily conceived, would not be sufficiently 

 separated. 



Although this experiment renders further proof of the cor- 

 rectness of my assertion superfluous, I will nevertheless intro- 

 duce a few others which have furnished me with the same 

 results. 



Upon the side of the prism turned towards the light, I laid 

 a piece of black paper, so that light could only be received 

 through the lens from below. The lens and prism being pressed 

 more forcibly together, the place of contact, when observed 



