OF MICROSCOPIC OBJECTS. 103 



laid upon it should be warmed and its surface covered with 

 a thin layer of balsam, and then lowered gradually upon 

 it, when no bubbles should be imprisoned. A narrow 

 piece of card-board at each end of the object, for the 

 cover to rest upon, prevents any danger of crushing the 

 specimen. 



This mode of mounting polypidoms, &c., seems to give 

 almost the complete beauty of the fresh specimens. They 

 are very beautiful objects when viewed with common light, 

 but much more so when the polarizer is used (in the manner 

 described a little farther on). 



To the above instructions there can be little to add ; but 

 I may here mention that some young students may not be 

 possessed of the air-pump, and on this account put aside all 

 search for those specimens which need little looking for at 

 the seaside. Many of these, however, though they lose some 

 beauty by the ordinary mode of drying, will, by steeping for 

 some time in turpentine, not only be freed from the air- 

 bubbles, but suffer so little contraction that they are a 

 worthy addition to the cabinet. 



Another class of objects is the spicula met with in 

 sponges, &c. These are often glass-like in appearance and 

 of various shapes ; many are found resembling needles 

 (whence their name) ; some from the synapta are anchor- 

 like, whilst others are star-like and of complex and almost 

 indescribable combinations. As some of these are composed 

 of silex and are consequently not injured by the use of nitric 

 acid, the animal substance may be removed by boiling them 

 in it. Those, however, which are calcareous must be treated 

 with a strong solution of potash instead; but whichever 

 way is used, of course they must afterwards oe freed from 

 every trace of residue by careful washing. 



These spicules may be often found amongst the sand 

 which generally accumulates at the bottom of the jars in 

 which sponges are kept by those who deal in them, and 

 must be picked out with a camel-hair pencil. The speci- 

 mens obtained by this means will seldom if ever require 



