in the Shadow of a Circular Disc. 153 



yet they are easily seen by such an eye-lens as in common ex- 

 perience we use in examining phaenomena of diffraction : 

 be it remembered these satellites are also bodies reflecting a 

 very feeble light. Further, Mr. Airy says, a spot whose dia- 

 meter is 0*0037 inch, or about 3^3 inch, is " a speck difficult 

 even for a philosopher to discover under these circumstances." 

 Now j^-q inch is the breadth of an ordinary hair of the hu- 

 man head, and these are to be seen of a perceptible breadth, 

 even as opake bodies surrounded by opaJce bodies : would not 

 a philosopher both see, and expect to see, bright spots when 

 surrounded by darkness, even if they were in diameter only 

 a very small fraction of this magnitude? 



To proceed to the immediate subject of the paper : I made 

 preparation for trying the experiment several months ago, 

 but when the apparatus was completed it was some time be- 

 fore a clear sunshine occurred to suit the aspect of the win- 

 dow of my darkened room, at a time when I could command 

 leisure to pursue the experiments. 



I prepared, by turning and polishing very carefully in a 

 lathe, discs of brass of y 1 ^, ^p t 3 q, t \, and T 7 ^ inch in dia- 

 meter, and with watch-makers' implements one of ^ inch 

 diameter. The luminous point was the sun's image formed 

 by a lens of £ inch focal length, and therefore it was of con- 

 siderably less diameter than t ^-q inch. The discs were at- 

 tached by thin films of cement to a plate of glass with flat 

 and parallel surfaces, which had never been before used, and 

 they were so formed that the diffracting circular edge was 

 raised from the glass ; in some of them this edge was formed 

 by the junction of the side and base of the frustum of a very 

 obtuse-angled cone, and in others the edge was formed by 

 the junction of two such frustums. The base by which they 

 were attached to the glass was in all of much smaller diame- 

 ter than the diffracting edge, so that the cement which oozed 

 out round the edge of this base, was far out of the way of 

 the light concerned in the phaenomena of diffraction. The 

 sun's light was reflected horizontally through a window-shut- 

 ter of a darkened room, and the sun's image, formed by the 

 lens of £ inch focal length, was at 60 inches distance from 

 the disc, at the same time that the focus of the eye-lens, by 

 which the phaenomena of diffraction were examined, was at 

 60 inches distance on the other side. The plate of glass to 

 which the discs were cemented, was rendered perpendicular 

 to the incident light by adjusting until the reflected light passed 

 directly back again. I generally used the apparatus so that 

 the plate of glass was between the disc and luminous point, 

 but I found no difference in the phaenomena when it was re- 



