OF MICROSCOPIC OBJECTS. 117 



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 

 41 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 da,rk slate-coloured fluid 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 

 potassse, 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 jifiLaffords 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 



