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- 
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 d f, and formed by the rays abc ef \ 
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 aijth 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 ?Vth 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 
Fig. 5. 
