periodical coloiirs produced by grooved surfaces. 57 



it a film of oil of cassia, I raised the blue to a gamboge yellow, 

 and I found that the fluid developed the phenomena of ob- 

 literated tints on the first prismatic image. Owing to the 

 great breadth of the spectrum, the distinct separation of the 

 colours which composed it, and the great leLgth of the line of 

 obliteration, this phenomenon was one of the most beautiful 

 and remarkable that I have ever witnessed. 



Hitherto I had examined the minima in the prismatic ima- 

 ges as symmetrically related in position to the minima in the 

 ordinary image, as shown in Figs. 10 and 11 ; but in studying 

 some specimens in which the spaces n were very broad, and 

 the grooves or spaces m comparatively narrow, I was surprised 

 to observe obliterated tints on the prismatic images, while the 

 ordinary image was entirely free of colour. This took place 

 in two specimens, one of which had 612, and the other 6^5 

 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 cut- 

 ting 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 observe a beautiful coincidence between the po- 

 sitions of the minima on the ordinary 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 per- 

 fect transparency of isinglass in thin plates, I have been ena- 

 bled 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 reflection three orders of colours, those seen by trans- 

 mission were only the following, 



