379 



relation to the defalcation of colour in the prismatic images. IB 

 order to observe these phenomena through a great range of inci- 

 dence, he employed a long narrow rectangular aperture, which gave 

 a convergent beam of 30 or 40. Under these circumstances, the 

 ordinary image of the aperture, as formed by the original surfaces, 

 was crossed, in a direction at right angles to its length, with broad 

 coloured fringes, varying in their tints according to the angle of in- 

 cidence. In a specimen having 1000 grooves in an inch, no less 

 than four complete orders of colours are developed, corresponding to 

 those of the reflected rings of thin plates. By turning the steel 

 plate round in azimuth, the same colours are seen at the same angles 

 of incidence, and they undergo no change by varying the distance of 

 the luminous aperture, or of the eye of the observer. 



The analysis of these curious and apparently complicated pheno- 

 mena is much simplified by the employment of homogeneous light. 

 The author pursues this analysis with red and with violet light re- 

 spectively, and explains the obliteration of the colours by the aid of 

 diagrams, giving also various tables of the angles of incidence at 

 which the several deficiencies occur in the reflected colours. These 

 angles are rendered different by covering the steel plate with water 

 and oil of cassia in succession. Phenomena analogous to those above 

 described take place on the grooved surfaces of gold, silver, calcare- 

 ous spar, and otiier substances. Similar grooves impressed upon tin, 

 realgar, and also upon isinglass, exhibited phenomena diversified ac- 

 cording to the respective refractive powers of these substances. The 

 almost perfect transparency of isinglass enabled the author to ex- 

 amine the transmitted tints, which in the ordinary image he found 

 were extremely brilliant, but had no relation whatsoever, either in 

 number or in quality, to the reflected tints. The transmitted tints 

 of the ordinary prismatic images always increase in brightness as the 

 angle of incidence diminishes ; while the reflected tints become 

 fainter. 



The new class of periodical colours described in this paper can- 

 not, in the opinion of the author, be referred to the diffraction and 

 interference of the rays reflected from two or more of the por- 

 tions of the original surface of the metal, considered as narrow slits 

 or apertures ; because they would in that case be affected by the 

 distance both of the luminous object and of the eye, and the colours 

 would form bands parallel to the direction of the grooves. But if we 

 suppose that the parts of the original surface are smaller than the 

 distance to which the reflecting force extends, the removal of the 

 metal from the adjacent grooves must diminish the reflecting force 

 of these parts of the surface ; and he infers, from direct experiment, 

 that this is the case. 



On the hypothesis of emission, this abstraction of reflecting matter 

 may be regarded as equivalent to a diminution of the density of the 

 surface ; while on the undulatory hypothesis, the effect may be as- 

 cribed to the condition of the ether arising from a variation in its in- 

 tensity or elasticity towards the surface of a number of salient points. 





