118 
Mr Hei>:chel on certain Optical Plce7iomma 
At No. 6, The axes of dispersion formed aii angle of 60°. 
At No. 7, They were 17° inclined to each other. 
In general, in dilFerenc specimens I could always predict, d 
'priori^ the situation of the nebulous masses of the second pair, 
by observing with the microscope the direction of the second set 
of grooves, to which they are invariably at right angles ; and the 
connection between the two phenomena is thus clearly demon- 
strated. Yet this connection is not, as in the case of the re- 
flected colours, that of cause and effect. The nebulous masses, 
as we have seen, subsist when the grooves (if any) are obliterat- 
ed by immersion in a fluid of equal refractive density. It is, 
therefore, in the internal structure of the pearl, that we must 
look for the common cause tf both appearances. When we ex- 
amine a thin plate of this substance by polarised light, the phe- 
nomena of a crystal with two axes of double refraction are ob- 
served ^ ; the isochromatic lines being perfectly regular, and si- 
milarly disposed in all parts of the surface, and the colours, 
though not vivid and somewhat hazy, yet following in their 
proper order, and extending sometimes to 6, 7, or 8 repetitions 
of the same colour from either pole If, now, we notice the 
situation of the axes of no double refraction, with respect to 
what has above been called the axis of nebulous dispersion, w6 
shall find, that the latter is in all cases at right angles, to the plane 
in which the former lie^ or to the optic meridian of the crystal. 
The nebulous masses, then, in all probability, originate in a 
regular laminated structure, perpendicular to the natural surface 
of the shell, and yniformly pervading all the coats of which it 
consists. The laminae, to agree with the above facts, must run 
* The systeftis of rings and the two axes of double refraction, wei'e discovered 
by Dr Brewster, and this is the only example of their occurrence in substances of 
animal and vegetable origin, excepting in the case of spermaceti,, where he also 
found them. See Phil Trans. 1814 p. 217,, and 1818 p. 230. — Ed. 
•J* Owing to the flexibility of mother-of-pearl, it is impossible to pi'ocure polish- 
ed pla:tes c‘f any regular thickness, w’hen below the 80th or lOOth of an inch, which 
is the thickness most convenient for these experiments. Hence it is indispensably 
necessary to examine but a very minute portion of the plate at a time (as of an 
inch), and to pass in succession over various parts of its surface ; otherwise the sys- 
tems of rings, corresponding to the different thicknesses at once under examina- 
tion, interfere with, and confuse one another, and might lead us to apprehend 
some considerable irregularity of ciystallisation where none whatever exists. 
