154- Mr. Potter on the Phenomena of Diffi-action 



versed, or the disc between the plate and the luminous point. 

 It was also found, on trial, that the appearances were not 

 sensibly changed if the focus of the eye -lens were rather more 

 or rather less than 60 inches from the disc. 



When the whole was adjusted, on looking through an eye- 

 lens of about 1 inch focal length, at the centre of the shadow 

 cast by a disc, there was seen a bright central spot, of a 

 white colour slightly tinged with brown, surrounded by a 

 greater or less number of coloured rings, according to the 

 size of the disc. The central bright spot in the shadow of 

 the disc of -^ inch diameter was large, and so bright, that at 

 the first view it would have been taken to be equally bright 

 with the light which had passed uninterruptedly, further from 

 the edge of the disc than any interference was visible. But 

 experience in photometrical experiments immediately pointed 

 out to me the allowance to be made for the heightening effect 

 of contrast, the bright spot being surrounded by a dark ring. 

 It was evidently necessary to place the portions of light, 

 which were to be compared, in the same circumstances. To 

 effect this, I perforated a plate of thin sheet brass with the 

 point of a needle, making a number of small circular holes 

 of different sizes and at different distances, so that they could 

 be taken in pairs of equal size. Now placing this brass plate 

 in the focus of the eye-lens, where it was moveable and ca- 

 pable of adjustment, and causing one of these holes to trans- 

 mit the central part of the central bright spot, and then com- 

 paring it with the light passing through an equal hole at a 

 distance beyond any visible interference, it was seen at once 

 that the brightness of the centre of the central bright spot in 

 the shadow was very much less intense than that of the light 

 passing through the other hole. In the shadow of the larger 

 discs the inferiority of the brightness of the central spot was 

 evident at once without any precautions, and the intensity 

 appeared to diminish rapidly with the increase of diameter of 

 the disc. The capability of the undulatory theory to explain 

 the phenomena is thus completely set at rest ; for, according 

 to it, the brightness of the centre of the shadow should be 

 equal to that of the uninterrupted light for all magnitudes of 

 discs. 



To obtain an estimate of the relative intensity of the light 

 of the central spot for the disc of ^ inch diameter, I adopted 

 the following contrivance : — Having cut a number of small 

 plates of mica out of the same sheet, I tried how many of 

 these placed before the uninterrupted light reduced its inten- 

 sity to an equality with that of the centre of the central spot, 

 when the brass plate perforated with the small circular holes 



