periodical colours produced by grooved surfaces. 59 



1. A specimen with 1000 grooves exhibited no colours on 

 the ordinary image either by reflection or transmission. The 

 prismatic images of a c^aildle Were very faint, arid the fourth 

 could scarcely be seen. 



% Another specimen of 1000 grooves gave by reflection 

 one period of colours from white at great incidences through 

 yellow up to purple at a vertical incidence. 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 reflection 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. 



As the steel plate from which all these impressions had been 

 taken was much injured, I resolved to grind down its surface 

 by a polishing powder, and to observe the changes which took 

 place. As the effect of this was to increase the spaces w, the 

 colours on the ordinary image soon disappeared. The pheilo- 

 itienon of the obliterated tints was no longer seen, the mass of 

 white light disappeared, and from the rounding of the edges of 

 the grooves the prismatic images were few^r in number, though 

 their distance was unchanged. 



Such are the leading phenomena of this n^w and remarkable 

 class of periodical colours ; but though their general law and 

 the circumstances upon which they depend seem to be pretty 

 clearly shown in the preceding experiments, yet I feel great 

 difficulty in assigning a satisfactory cause for their production. 

 That they are not owing to the diff'raction and interference of 

 the rays reflected from two or more of the surfaces n, consider- 

 ed as narrow slits or apertures, is obvious; for in that case 

 they would be affected by the distance of the luminous object 

 and the distance of the eye, and the colours would form bands 

 parallel to the direction of the grooves. 



In my experiments on the production of the complementary 

 colours by the metallic reflectibn of polarized light, I have 



