eochiUted hy Motlier-of-Pea/rl. 117 
ccmd set of grooves, denoting those observed by Br Brewster 
by the first set), with the nebulous masses above described, I 
chose a plate in which the first set varied from the extreme of 
coarseness to that of delicacy, and were particularly irregular in 
their direction and curvature. In this I carefully marked, by 
small ink circles, several spots, numbered 1, 2, 8, 4, 5, 6, 7, 
which were then subjected to microscopic examination. 
At No. 1 and 2, The two sets of grooves coincided in direction. 
At No. 3, They made an angle rather more than 45° by the 
eye. 
At No. 4, No grooves of the first set could be seen, but a 
power of 229 shewed some obscure and very irregular traces 
of coarse elevations and depressions. With this power, how- 
ever, the second set were seen precisely as in every other part 
of the surface, and in the same uniform direction. 
At No. 5, The first set, as they approached this point, grew 
smaller and smaller, requiring powers of 26. B4. 48. 128. in suc- 
cession, to perceive them. With this latter power the second 
set just became visible ; while with 229, both sets were seen 
crossing each other at right angles, with the most perfect regu- 
larity and distinctness, the former being about twice the breadth 
of the latter. 
At No. 6, The direction of the first set varied a good deal, 
crossing the second from 45° to 60°. 
At No. 7, They crossed at a small angle. 
I now detached the mother-of-pearl, and by passing a small 
sun-beam successively through each of the marked spots, and 
noticing the relative situations of the two pairs of coloured 
images, it appeared that 
At No. 1 and 2, The axes of dispersion in the first and second 
pair of coloured images were coincident. 
At No. Sj’^They made an angle of 45° with each other. 
At No. 4, There were no coloured images of the first pair 
whatever, while those of the second pair had precisely the same 
appearance and direction as in the other parts' of the plate. 
At No. 5, The axes of dispersion formed a right angle, the 
images of the first pair being very vivid, and separated by aq 
unusnally large interval. 
