COLOURS OF THICK PLATES. [149] 



silvered glass, placed at the distance of some feet. This observation was communicated 

 to M. Quetelet, by whom it has been published*. In repeating the experiment together, 

 Dr Whewell and M. Quetelet found that it was an essential condition of success that the surface 

 should not be perfectly bright, and that to ensure the production of the bands it was sufficient 

 to breathe gently on the surface of a cool mirror. Instead of vapour, which soon evaporates, 

 M. Quetelet recommends a tarnish of greasej-. 



In closing this sketch of the history of the subject, I may be allowed to express my 

 obligations to Dr Lloyd for his valuable Report on Physical Optics, which contains a brief 

 account of all that was known about the subject, accompanied by references to the original 

 papers. 



My attention was called to the subject by the Master of Trinity College, who shewed me 

 the bands above-mentioned, which he shortly afterwards brought before the notice of this 

 Society \. It seemed to me from the first that these bands were of the nature of the coloured 

 rings of thick plates, so that the theory of the former only required to be worked out, that of 

 the latter being known. Had I felt any doubt on the subject, it would soon have been dissi- 

 pated when I came to make experiments ; for by properly varying the experiments the two 

 systems were seen to be incontestably of the same nature. 



The mirrors, whether plane or curved, were prepared in the following manner, which I 

 can recommend to any one who wishes to repeat the experiments, as being both easy and 

 efficacious. The mirror being held horizontally, a mixture consisting of three or four parts 

 of water to one of milk was poured on it, and allowed to spread over the surface. The mirror 

 was then held in a vertical position in front of a fire, when the greater part of the mixture 

 ran off, and the remainder dried in two or three minutes, when the mirror was ready for use. 

 To prevent disappointment on the part of any one who may be looking for Dr Whewell's 

 bands, I will here mention that in order to see them properly tbe image of the flame must be 

 seen distinctly, so that a shorted-sighted person requires an eye-glass or spectacles. 



A concave mirror prepared with milk and water is well adapted for performing Newton's 

 experiment in his manner, or rather by substituting, as in the Duke de Chaulnes's experiments, 

 the image of the sun in the focus of a convex lens for the small hole employed by Newton. 

 The experiment may however be varied in the following manner. Whatever appearance is 

 presented on a screen may be seen without a screen by receiving the rays directly into the eye, 

 and adapting it for distinct vision of an object at the distance of the screen. Accordingly, in 

 order to see the rings which in Newton's experiment were thrown on a screen, it is sufficient 

 to place a small flame in front of the mirror, in such a position as to coincide with its inverted 

 image, when a remarkably beautiful system of rings is seen in air, surrounding the flame. 

 Not the least striking circumstance connected with these rings is their apparent corporeity, 

 since they seem to have a definite position in space like an actual object. The striking and 

 beautiful phenomena so accurately described by Newton in his 10th and Hth Observations 

 may be seen in this manner by moving the flame sideways. By altering in various ways the 



* Correspondence Mathematique et Physique, Tom. v. | + Tom. vi. p. 69. 

 (1829) p. 394. j See the Philosophical Magazine for April, 1881, p. 336. 



