312 DR. BREWSTER ON PERIODICAL COLOURS PRODUCED BY THE 



image was entirely free of colour. This took place in two specimens, one of 

 which had 312, and the other 625 grooves in an inch. The spaces n were here 

 far too wide to produce the new tints, and so were the spaces m ; but upon 

 applying the microscope to the grooves m, I saw that they were formed by two 

 or more grooves ploughed out by the cutting point ; so that each space m 

 actually consisted of smaller reflecting spaces, which were sufficiently minute 

 to produce the periodical colours. 



Although in these specimens, therefore, when m is nearly equal to n, we ob- 

 serve a beautiful coincidence between the positions of the minima on the ordi- 

 nary and on the prismatic images, yet the fact above described seems to show 

 that they are separate phenomena, and depend, when the grooves are single, on 

 the relation between m and n. 



The preceding observations relate solely to rays reflected from grooved 

 surfaces ; but in consequence of the almost perfect transparency of isinglass in 

 thin plates, I have been enabled to examine the transmitted tints. The 

 colours which are thus seen on the ordinary image are extremely brilliant, but 

 they seem to have no relation whatever, either in number or in quality, to the 

 reflected tints. In the specimen which gave by reflexion three orders of colours, 

 those seen by transmission were only the following. 



o 



Fine blue 85 of incidence. 



Purple. 



Red. 



Orange. 



Yellow vertical incidence. 



Another specimen from the same steel plate gave, when soft and newly taken 

 off", a bright purple at a perpendicular incidence, which passed through pink 

 and blue at greater incidences. But in the process of induration, the vertical 

 purple became red, orange and yellow. In a third impression the perpendi- 

 cular tint was a bright pink when soft, which descended to yellow when drier. 

 In order to observe the relation between the reflected and transmitted tints, 

 I took a fresh impression on very transparent isinglass, and obtained the fol- 

 lowing results. 



