20 PREPARATION AND MOUNTING 
Camphor dissolves in a only very small proportion in water, 
but freely in alcohol, ether, and strong acetic acid. 
9TH DIVISION. 
It is by no means intended to speak here of the general 
properties and uses of polarized light. But in relation to 
its 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 
