Mr. Potter's Reply to Professors Airy and Hamilton. 279 



Pr. Young's article " Chromatics " in the Supplement to the 

 Encyclopaedia Britannica. Dr. Young there gives as follows : 

 " In reflections at the surface of a rarer medium, and of some 

 metals, in all very oblique reflections, in diffraction, and in 

 some extraordinary refractions, a half interval appears to be 

 lost." 



Dr. Young most probably wrote the above before M. Fres- 

 nel had adopted his new theory of diffraction, and had at- 

 tempted to disprove his first view. It will be found, however, 

 in his still later writings (see Quarterly Journal of Science, 

 &c. for 1827, p. 4.50.), that Fresnel did not entirely abandon 

 the theory of the light reflected at the edges of bodies pro- 

 ducing diffracted fringes. As to his experiments to determine 

 between his two views, I believe I have only need to object to 

 their sufficiency, — that the red light given by the glass from 

 the windows of old churches is quite insufficiently homogene- 

 ous where so small differences are to be ascertained ; and this 

 was the light which he made use of. 



That the undulatory explanation of Newton's rings is inad- 

 missible we may infer from the following fact: If we press a 

 lens against one side of a cube of glass, on looking through 

 the opposite side we see the central dark spot surrounded 

 with the rings : if we look through one of the adjacent sides 

 we see the central black spot, but without the rings, in the 

 midst of a surface giving total reflection. In this latter case 

 it will be impossible to account for the black centre by inter- 

 ference; and the same solution must apply to it which applies 

 to the other case. When the lens is of less refractive glass 

 than the cube, and the light is incident on the second surface 

 of the glass cube at the critical angle lor the two surfaces, the 

 dark spot, after taking various tints, commencing with purple, 

 entirely disappears. The best mode to observe the pheno- 

 menon above offered to notice, is to press the lens against the 

 hypothenusal side of a glass prism having two angles of 45 

 degrees each: we can then see the black spot either in the 

 totally reflected light, or in the partially reflected light, by 

 slightly elevating or lowering the eye ; and we see that the 

 white of the first ring is the point where total reflection first 

 becomes perfect, and from which we ought to commence our 

 measurements of the spaces between the surfaces and our cal- 

 culations of interference. 



The question of half an undulation is more directly and 

 effectually to be settled with the simple experiment of the two 

 mirrors. Those who maintain the correctness of the undula- 

 tory theory, invariably assure us that the central band is always 

 a white one. The result of considerable experience with me 



