OF MICROSCOPIC OBJECTS. ey, 
Few objects are more beautiful with polarized light 
than young oysters. Good colours and a decided cross 
are given by them when well prepared. The following is 
the method pursued by Mr. Henry Lee :—Having found a 
“ black-sick ” oyster (to use the dredgemen’s term), the spawn 
of which is quite mature and ready for extrusion, pour off 
from the shell the dark slate-coloured fiuid into a long 
narrow two-ounce phial; fill up the bottle with distilled 
water; shake it gently; allow the deposit to settle, and 
change the water two or three times, repeating the agitation 
to get rid of the salt. Then substitute for the water liquor 
potassz, diluted with equal quantity of distilled water. 
Allow the young oysters to remain in this for two days, 
_ agitating occasionally ; and as often as the liquid becomes 
discoloured pour it off, and renew the same until no colour 
is given off and the shells are seen to be thoroughly cleansed 
from all animal matter by their sinking freely and rapidly 
to the bottom. When this stage is arrived at, stop the 
process, that the two valves of the shells may not be sepa- 
rated by the destruction of the hinge. Wash repeatedly in 
distilled water, to remove all trace of alkali, and finally wash 
and preserve the shells in a little rectified spirits of wine 
(not methylated spirit). These objects are frequently 
mounted in balsam, to increase their transparency for the 
polariscope, but where they are sufficiently clear they may 
be mounted dry like the foraminifera. 
The scales of various fish have been before mentioned 
as mounted dry; when, however, they are required for 
polarizing objects, they are generally mounted in balsam, 
and some few in liquid. The former method will be con- 
sidered here. 
The eel affords a beautiful object for this purpose. The 
scales are covered by a thin skin, which may be slightly 
raised with a knife and then torn off, in the same manner 
as the covering of the geranium and other petals, described 
in Chapter III. The required portion may then be removed ; 
or if a piece of skin can be procured as stripped off in 
