QQ REPORT ON PHYSICAL OPTICS. 



be ranked the analysis of the colours of mother-of-pearl,* and the 

 investigation of the structure of the crystalline lenses of the eyes 

 of fishes and other animals, by Sir David Brewster.f The same 

 author has also described a new series of periodical colours, which 

 are exhibited by some of the plates of grooved steel constructed by 

 Mr. Barton, and which succeed one another in a plane at right 

 angles to that in which the usual spectra are developed.? The 

 theory of this phenomenon remains yet to be developed. In the 

 solution of the analogous problem, given by Professor Airy, a 

 periodical variation in the intensity of the light in the direction of 

 the apertures of the grating is indeed pointed out ; but that varia- 

 tion, it is easily seen, will not account for the facts last mentioned. 



IV. Colours of thin Plates. 



The earliest observations on record, in which the colours of thin 

 plates were made the subject of experimental research, are those 

 of Boyle. This diligent observer remarked the fact, that most 

 transparent substances exhibit colour by reflected light when suf- 

 ficiently reduced in thickness ; and that these tints varied in the 

 same substance, and therefore did not depend essentially upon its 

 chemical nature. The observations of Boyle were made on the 

 bubbles of various liquids, and he even succeeded in blowing glass 

 sufficiently thin to exhibit similar phenomena. 



The vivid and varying colours of the soap bubble also engaged 

 the attention of Hooke ;|| but the most important of the observa- 

 tions of this philosopher, connected with the subject of thin plates, 

 are those recorded in his Micrographia, which was published in the 

 year 1665. In this work he shows that the colours of laminae of 

 mica are dependent on their thickness, and appear only when that 

 thickness is comprised within certain limits; that when the tint 

 exhibited by a given plate is uniform over its entire surface, the 

 plate is also uniformly thick ; and that the colour presented by 

 two plates superposed is different from those of either beparately. 

 Hooke has also the merit of producing the phenomena of thin 



* Phil. Trans. 1814. 



f Ibid. 1833. 



J Phil. Trans. 1829. 



Experiments and Observations upon Colours, 1663. 



II Birch's History of the Royal Society, vol. iii. p. 29. 



