EXAMINATION AND PRESERVATION MEDIA. 283 



The risibility of minute structures is proportional to the 

 <Hn'' rence between the refractive indices of the object and of 

 the medium in which it is mounted. The majority of the 

 elements of soft tissues are (after fixation) of an index of 

 refraction somewhat superior to that of Canada balsam. It 

 follows that by lowering the index of the balsam, increased 

 visibility is obtained, and the desideratum in any case is to 

 find a medium just low enough to give good visibility, and 

 yet not so low as to seriously cut down the N.A. of the objec- 

 tives employed. 



439. Choice of a Mounting Medium. For the foregoing 

 reasons I recommend turpentine colophonium for general 

 work, with the restrictions mentioned, 443 ; whilst for 

 cases in which a more highly refractive medium is desired, 

 I would recommend oil of cedar or xylol-balsam for coal-tar 

 stains. Xyiol-balsam is certainly a very fine medium. I have 

 merely wished to point out that it is not perfectly safe, on the 

 score of the possible formation of granules. (P. MAYER, in 

 Utt., is of the same opinion.) I also recommend for carmine 

 or iron-haematoxylin stains SEILER'S alcohol-balsam (MAYER 

 prefers VOSSELER'S turpentine, 444). For haemalum stains, 

 xylol-balsam. 



440. Canada Balsam. Prepare with the solid balsam as 

 above described, 438. The usual menstrua are xylol, 

 benzol, chloroform, and turpentine. Dissolve the solid balsam 

 in one of these to the required consistence. The turpentine 

 solution is to be preferred only in cases where it is desired 

 to have a medium that sets very slowly. (The objection to 

 turpentine as a solvent is that it does not always give a 

 homogeneous solution with Canada balsam as it does with 

 colophonium.) For most other purposes the xylol solution is 

 the best. If time be an object, a benzol solution should be 

 preferred, as it sets much quicker than the xylol solution. 



SAHLI (Zeit. f. u-iss. Mik., ii, 1885, p. 5) dissolves in cedar 

 oil. 



Samples of balsam that are acid are frequently met with, and are 

 injurious to some stains. Griibler & Hollborn now prepare a neutral 

 balsam, in which MAYER has found that very delicate preparations, that 

 lost colour immediately in any other sort of balsam, have kept perfectly for 



