POLARIZED LIGHT. 117 



The palates of mollusca, as polariscope objects, 

 are not as a rule very effective, that of the common 

 Whelk excepted. 



Hairs are worthy of notice for polarizing pur- 

 poses, as they usually display a considerable amount 

 of colour. White horsehair and grey human hair 

 (Fig. 5, pi. 9) are perhaps the best for the student's 

 purpose. 



The structure of Rhinoceros horn and whale- 

 bone is well displayed when polarized, but they 

 are difficult to prepare, and it would be better to 

 purchase them of the dealers in microscopic pre- 

 parations. 



The mineral kingdom affords but few objects 

 that can be prepared by the amateur, although no 

 collection of polariscope objects would be com- 

 plete without one or more sections of agate and 

 chalcedony; and, like the objects previously 

 named, must be obtained from the dealer. The 

 young microscopist should, however, obtain from 

 some optician a piece of the so-called Brazilian 

 pebble (really transparent quartz), and break it 

 up into small fragments : many of these, when 

 mounted, display very beautiful coloured rings. 



A few words may perhaps be necessary as to the 

 mode of procedure in mounting specimens of 

 crystal. We have in several instances directed 

 the solution to be placed on the cover ; our reason 

 for doing so is, in order to avoid the application 

 of any great degree of heat, and at the same time 

 using tolerably hard balsam. 



The plan we adopt is as follows. Place a drop 

 of pure balsam on the centre of a slide, harden 

 over the lamp (it will be sufficiently so if the 

 nail slightly indents it when cold) ; now drop a 

 little turpentine on the prepared cover, holding it 

 as close as possible to the edge with the forcepa. 



