GROOVED SURFACES OF METALLIC AND TRANSPARENT BODIES. 313 



Reflected tints. Transmitted tints. A ngles of incidence. 



Yellow Deep blue .... 90 



Orange Paler blue. 



Pink Blue. 



First limit of pink and blue . . Blue. 



Blue Pink. 



Green Orange pink. 



Yellow Orange. 



Orange Yellow. 



Pink Yellow. 



Second limit of pink and blue . Yellow. 



Blue Yellow ..... 



The comparison of these tints affords the most satisfactory evidence that they 

 are not complementary to each other. The transmitted tints of the ordinary 

 prismatic images always increase in brightness as the angle of incidence di- 

 minishes, while the reflected tints become fainter. 



As I had preserved the different specimens of isinglass with which these ex- 

 periments were made, it became interesting to observe the changes which their 

 colours had undergone after a lapse of six years. The following was the 

 result. 



1. A specimen with 1000 grooves exhibited no colours on the ordinary image 

 either by reflexion or transmission. The prismatic images of a candle were 

 very faint, and the 4th could scarcely be seen. 



2. Another specimen of 1000 grooves gave by reflexion one period of colours 

 from white at great incidences through yellow up to purple at a vertical in- 

 cidence. By transmission a little yellow only was seen at a great incidence. 



3. A third specimen of 1000 grooves which had been a fine sharp impression, 

 gave by reflexion two orders of colours, the first limit of pink and blue being 

 at 57° 45', and the second limit nearly at a vertical incidence, a deep pink 

 appearing at 10°. By transmission the isinglass gave a bluish green at the 

 greatest incidence which passed at lesser incidences through purple to yellow, 

 which was the maximum tint. 



In all these specimens the colours remain the same in all azimuths, provided 

 the angle of incidence is invariable. 



MDCCCXXIX. 2 s 



