120 Mr Herschel on certain Optical Phenomena 
of a polished plate of mother-of-pearl, in which the edges of the 
laminae, reduced to the utmost tenuity, by the effect of their in- 
clination to the general surface, are torn up in their direction of 
least resistance, by the action of the polishing particles. This 
is rendered perfectly evident by the microscopic examination of 
the surface, in different stages of its progress from the rough 
grinding to the most .perfect polish, when the grooves, from an 
irregular, jagged, and deeply indented outline, will be seen to 
assume a greater and greater neatness of termination, till their 
curvature acquires that graceful and flowing character which 
ultimately distinguishes them. See Figs 3. 4. 2. 
I must here take an opportunity to remark, that Dr Brew- 
ster’s conclusion, as to the existence of a second reflecting force 
below that which produces the ordinary image, does not appear 
to me to rest on sufficient evidence. Were it indeed true, that 
the whole face of a polished plate of mother-of-pearl consists of 
elevations and depressions, with little or no intermediate plane 
surface, his conclusion would be irresistible ; but, in all the mi- 
croscopic observations I have had occasion to make, such has 
not appeared to me the case ; the flat interval between two 
grooves bearing always a sensible proportion to that curvilinear 
depression which constitutes the groove itself, and affording a 
surface whose section may be represented by something like 
Fig. 5., where the portions ah^ ah, are true planes on which 
the ordinary reflection may take place ; and as each groove 
bears a very small proportion in breadth to the diameter of the 
pupil of the eye, the effect of the intervals be a, he a, is merely 
to abstract a certain uniform portion of light from regular re- 
flection, and disperse it according to their own peculiar consti- 
tution and figure. This disposition is very distinctly visible in 
the impression of the surface taken on rosin, though less so in 
the pearl itself (unless in the first stages of its polishing), from 
the dispersion of the light in its interior, which renders micro- 
scopic observations on it peculiarly liable to illusion ; while in 
the impression on rosin, each groove is seen with admirable dis- 
tinctness, and its shape and the inclination of its sides may be 
clearly traced. 
Besides the two nebulous masses above described, which lie 
parallel to the optic meridian of the pearl, two others;^ very 
