116 Mr Herscliei cm cei^tain Optical Phenomena 
Gohnected with any peculiarity in the superficial structure, and 
I scarcely expected the application of the microscope to afibrd 
farther information, or lead to any result worth notice. My 
surprise, therefore, was extreme, when, on examining every po- 
lished specimen of mother-of-pearl in my possession with a 
powerful double microscope, I found that this extraordinary 
body, in addition to the irregular grooved superficial structure, 
1 ^ beautifully described by the above-mentioned philosopher, 
possesses (at least to all appearance) another of great regulari- 
ty and delicacy ; but, like the former, resisting every attempt to 
impair it by polishing the surface. It may be seen to most ad- 
vantage on a thin polished specimen, in which the first set of 
undulations vary a good deal in direction and coarseness. When 
we view such a plate successively with a series of increasing 
magnifiers, under a double microscope, a power of 12S will 
barely shew, and one of 229 completely verify, the appearance 
of a minute system of rounded undulations, consisting, as it 
were, of fibres occasionally branching from each other, but 
never continued for any length. They are uniformly diffused 
over the whole surface, and, in their general direction, disposed 
in strait and exactly parallel lines, running from one end to the 
other, (See Plate III. Fig. 1.) In consequence they cross the first 
set of grooves at all angles, giving the whole surface much the 
appearance of a piece of twilled silk, or the larger waves of the 
sea intersected with minuter ripplings. Fig. 2. is an imperfect 
attempt to represent this appearance. 
The interval between these undulated appearances is nearly 
die same in all the specimens I have examined. To ascertain 
it, twenty-five were counted in the space of an inch, in an image 
projected on a plane 10 inches from the eye, while the diameter 
of a small wire, projected on the same plane with the same 
power, measured 7 inches. The diameter of this wire, taken by 
in in 
the sphaerome^er, being 0’0227, gives 0.000129, or of 
an inch for their mutual distance. To see them distinctly, a care- 
ful management of the illumination from below is necessary, 
and candle-light must be used. 
To demonstrate the connection of these undulations, (which, 
whether real or only apparent, I shall for brevity, call the se- 
