314 DR. BREWSTER ON PERIODICAL COLOURS PRODUCED BY THE 



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 n, the colours on the ordinary image soon disappeared. The phe- 

 nomenon 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 fewer in number, though their distance was unchanged. 



When one of the impressed films of isin- ^'s- s. 



glass mn, Fig. 5, was laid upon a plate of 

 glass A B C D, and was in optical contact 

 with it, a series of fringes was seen across 

 the images reflected from the second surface 

 C D of the glass. These fringes seen by the 

 eye at df, and formed by the rays ahcef, 

 are parallel to the grooves on the isinglass, 

 and their breadth diminishes as the thickness A C of the glass is increased. 

 When the grooves were 1000 in an inch, these fringes were nearly as distinct 

 as the prismatic images, one fringe appearing to bisect each image when the 

 thickness A C was about ^Vth of an inch. They were much more numerous, 

 and even crossed the principal image when A C was fth of an inch ; but when 

 A C was ^th of an inch, no fringes were seen across the second image. 



These fringes have the same origin as those which I have described in the 

 Edinburgh Transactions. In the first specimen, where A C was ioth. of an 

 inch, its two surfaces were not parallel, and the direction of the grooves in 

 the isinglass was accidentally perpendicular to the common section of the two 

 surfaces of the glass. Hence the fringes produced by the glass were parallel 

 to the prismatic images from the isinglass. But when the specimen is turned 

 round, the isinglass fringes reflected from the back of the glass are crossed by 

 those produced by the glass, giving to the former the appearance of a coloured 

 rope, in which the coils pass along the longitudinal spectra with singular 

 beauty. 



Such are the leading phenomena of this new and remarkable class of perio- 

 dical colours ; but though their general law and the circumstances upon which 

 they depend seem to be pretty clearly shown in the preceding experiments, yet 



