158        ON THE COLOURS OF THICK PLATES.
the eye is viewed by reflexion in a plane mirror of quicksilvered 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 grease j-.
| •                               In closing this sketch of the history of the subject, I may be
i                            allowed to express my obligations to Dr Lloyd for his valuable
ij                            Report on Physical Optics, which contains a brief account of all
that was known about the subject, accompanied by references to !j                             the original papers.
Ij                                  My attention was called to the subject by the Master of Trinity
^rj     '                        College, who shewed  me the bands above-mentioned, which he
'j                            shortly afterwards brought before the notice of this Society:]..    It
°|                            seemed to me from the first that these bands were of the nature
,j                            of the coloured rings of thick plates, so  that the  theory of the
j                            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
h                            been dissipated when I came to make experiments; for by properly
;j                            varying the experiments the two systems were seen to be ineon-testably 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 ellicacious.   The
mirror being held horizontally, a mixture consisting of three or four parts of water to one of rnilk 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 the image of
* Coireapondance Mathiimatique. ct Physique, Tom. v. (1H^12^(J) p. :{<M.
t Tom. vi. p. G9.
ij: See the Philosophical Magazine for April, 1851, p. ,'J,'J(J.