20 PREPARATION AND MOUNTING 



Camphor dissolves in only a very small proportion in water, 

 but freely in alcohol, ether, and strong acetic acid. 



9iH DIVISION. 



It is by no means intended to speak here of the general 

 properties and uses of polarized light. But in relation to 

 : ts special powers in the " differentiation " of tissues, there 

 is very much to be learned. To be fitted for examination 

 by this method, objects must be made more or less trans- 

 parent or translucent; and in effecting this it is advisable, 

 perhaps necessary, to employ media of high refractive power. 

 Even when so prepared, it- may be further necessary in some 

 cases to employ selenite or mica films, still more to enhance 

 their colour. Not the least indication can be afforded to the 

 observer as to what colours he should employ generally, yet 

 it is a matter of frequent observation that what are called 

 " neutral tints '' are to be preferred, such as result from the 

 judicious use of compound selenite stages adjusted properly 

 for that special effect. 



The media most suitable for the preparation of objects to 

 be examined in this way are glycerine, syrup, turpentine, 

 dammar and benzole or the latter alone, Canada balsam, 

 and the essential oils. Of course sections must be made of 

 tissues otherwise too thick. Of the advantages of employing 

 the first of these we have already spoken ; but to these must 

 be added this important one, that it does not spoil the object 

 for examination by other methods, if the glycerine be soaked 

 out by maceration in water ; and this is true also of syrup, 

 though it is far less useful. For preparation by the other 

 methods, tissues must have been soaked in alcohol, and then 

 removed into the turpentine, &c. 



For the examination of insects by polarized light, two 

 preliminaries are necessary. Firstly, that they be made 

 transparent or translucent by prolonged soaking in one of 

 the above-named media, preferably in turpentine or the 

 essential oils, or benzole. Secondly, that as in (most ?) 

 many of them their chitinous case is too deeply coloured for 



