in the Shadow of a Circular Disc. 155 



was used. From the result of many trials I concluded that 

 four plates produced the most accurate correspondence. To 

 find how much light these mica plates transmitted, I looked 

 through a rhomb of Iceland spar, at a strip of white paper so 

 broad that the two images overlapped, and of course at such 

 overlapping the intensity was the double of what it was on 

 either side of it: a similar effect was produced by somewhere 

 between two and three of the mica plates. I estimated that 

 it was nearer two than three plates which were required ; but 

 to a scientific friend, with whom, on another occasion, I re- 

 peated it, it appeared to be nearer three than two that were 

 requisite, so that I shall consider 2\ of the mica plates as 

 transmitting only one half of the light : and four such plates 

 would therefore transmit one-third part very nearly. This 

 shows how widely the fact was at variance with the result 

 of the undulatory theory even with a disc of A* inch dia- 

 meter. 



To perform these experiments the eye-lens and brass plate 

 in its focus were mounted on a stand so as to be raised or 

 lowered at pleasure, and the mica plates, mounted in bundles 

 of two, three and four, were attached to a rod of wood, so 

 as to be interposed in the light in succession without incon- 

 venience. 



I have also the following memoranda : in the shadow of the 

 J^ inch disc, besides the central spot, two rings around it were 

 visible ; in that of the ~ inch disc, four rings were visible; in 

 that of the T 2 n inch disc, five or six were visible ; and in that 

 of the T 3 y inch disc, seven or eight were visible : and also as 

 the diameter of the disc was increased, the intensity of the 

 rings became more nearly equal to that of the central spot. 

 The discs of T 4 ^ and T 7 ^ inch I did not use, as I must have 

 provided larger plates of glass to attach them to, for the edge 

 of the plates of glass to which the latter disc was attached 

 caused interference sufficient to distort the rings when this 

 plate was two inches square. 



I must be allowed to state, that I consider the controversy, 

 as to the undulatory theory being the physical theory of light, 

 to be nearly terminated ; and that the experiments necessary 

 for completing the basis of a physical theory are those now 

 most desirable to be undertaken. 

 Queen's College, June 9, 1841. 



