periodical colours produced by grooved surfaces. 55 



In these experiments the tin gave nearly the same results 

 as the steel ; but in the realgar and the isinglass, similar tints 

 were produced at a less angle of incidence than in the steel. 

 The minima of the periods were exhibited very finely on the 

 isinglass, and were produced at similar angles of incidence. 



In a specimen with 1000 grooves upon isinglass, the third 

 pink, or that seen upon steel at 36°, was the highest ; but after 

 drying, the pink descended to yellow, and subsequently to 

 green. 



If the isinglass is removed from the steel when it is still soft, 

 the edges of the grooves get rounded and lose their sharpness, 

 and only one prismatic image is seen on each side of the ordi- 

 nary image, as in mother-of-pearl. 



The mass of white light is finely seen in\the impressions taken 

 upon tin, but never appears upon isinglass. 



The preceding experiments do not afford any precise data 

 for determining the influences of refractive power. The real- 

 gar and the isinglass give fewer periods of colour so as to in- 

 dicate that, ccBteris paribus, a diminution of refractive power 

 produces a diminution in the number and orders of colours, or 

 causes the minima to be developed at a less incidence. This 

 indication, however, is opposed by the fact, that, as the isinglass 

 dries, and consequently increases in refractive power, the periods 

 diminish in number, and the minima are produced at less in- 

 cidences. The modification of the tints by a change of refrac- . 

 tive power is here masked by the influence of other causes, 

 namely, an inferiority in the sharpness of the impression to that 

 of the original surface, and a rounding of the narrow spaces n 

 subsequently produced by induration. In the specimen of 

 isinglass, therefore, already mentioned, which gave the first 

 limit of pink and blue at nearly the same angle as the steel, 

 it is probable that it would have developed the same limit at a 

 greater inclination had the impression been as sharp as the 

 original. 



In this uncertainty I conceived that the influence of a vari- 

 able refractive power would be best obtained by placing dif- 

 ferent fluids on the surface of the grooved steel; and upon 

 using alcohol and oil of cassia my expectations were fulfilled. 



The following were the results: — 



