Il l- Mr Herscliel on certain Optical Plmioinena 
Art. XX. — -On certain Optical Phenomena exhibited hy 
Mother - of - Pearl, depending on its internal structure. 
By J. F. W. Herschel, Esq. F. R. S. &c. Communicated 
bv the Author, 
In repeating the experiments of Dr Brewster on mother-of- 
pearl, described in the Philosophical Transactions for 1814, 
Part II., I observed some new phenomena exhibited by that 
singular body in its action on transmitted light, depending on 
the internal arrangement of its molecules, and at the same time 
connected with a peculiarity in its superficial appearance under 
the microscope, which seems to have eluded his notice. When 
a plate of mother-of-pearl, cut parallel to the natural surface of 
the shell, is reduced by grinding to a thickness between y’^th and 
^J^th of an inch, and highly polished on both sides, (in which 
circumstances it is very transparent) ; if a distant candle be 
viewed through it, besides the pair of coloured images describ- 
ed by Dr Brewster, which have the same origin with those seen 
by reflection, there may be observed two large very brilliant and 
highly coloured nebulous masses, one on each side of the candle, 
and equidistant from it, which may readily be distinguished 
from the preceding by the following characters. 
The first pair of coloured images, originating in the transfe- 
rable superficial structure of the pearl, are always similar in po- 
sition and colour, and complementary in brightness, to those 
seen by extraordinary reflection. In consequence, nothing can 
be more capriciously irregular than their situation, brightness, 
and distance from the centre. On passing various parts of the 
plate, with a parallel motion, between the pupil of the eye and the 
candle, they will be seen to shift their direction, expand, contract 
or disappear altogether, with every change in the point examined. 
This is not the case with the pair of nebulous masses now un- 
der consideration, which undergo little or no variation in any of 
these particulars, through whatever portion of the plate they 
are viewed. The axis of nebulous dispersion then, (or line joint- 
ing the two nebulae,) is parallel to itself, or nearly so, through- 
out the whole extent of the mother-of-pearl, and the energy of 
the cause producing it nearly or perfectly uniform. 
