338 FOURTH REPORT — 1834. 



ceed 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 variation, 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 Boylef. This diligent observer remarked the fact, that 

 most transparent substances exhibit colour by reflected light 

 Avhen sufficiently 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 suc- 

 ceeded in blowing glass sufficiently thin to exhibit similar phe- 

 nomena. 



The vivid and varying colours of the soap bubble also engaged 

 the attention of Hooke J ; but the most important of the obser- 

 vations of this philosopher, connected with the subject of thhi 

 plates, are those recoi-ded in his Microgrnphia, which was piib- 

 lished 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 tvvo plates superposed is different from those 

 of either sepai-atel)^ Hooke has also the merit of producing the 

 phenomena of thin plates in the instructive form in which their 

 laws have since been studied, namely, by placing two object- 

 glasses in contact; and he found that any transparent fluid in- 

 troduced between the lenses furnished a succession of colours 

 as well as air ; — the colour, however, being more vivid, the more 

 the refractive power of the plate differed from that of the glasses 

 within which it was inclosed. 



The attention of Newton was soon after directed to the same 

 subject ; and his investigations, which ended in the complete 



* Phil. Trans. 1829. 



f Experiments and Observations upon Colours, 1663. 



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



