146 SELECTION AND TJSE 



scribed, it is necessary to immerse it in some suitable medium, 

 so as to render it clear and transparent. The action of such 

 media may be very well illustrated by the following experiment: 

 Take a short piece of black human hair, place it on a slide, 

 bring it into focus and examine it. It will appear as a dark 

 cord with a light line running down the centre, and from this 

 circumstance has arisen the erroneous popular idea in regard to 

 the tubular structure of hair. Apply a drop of glycerine di- 

 lated with an equal bulk of water, and again examine it. The 

 appearance will have entirely changed, having become clearer 

 and more definite, so that the structure of the hair is more 

 easily made out. This effect depends upon the refracting power 

 of the liquid used. The following liquids are usually employed 

 for this purpose, their efficiency being in direct ratio to their 

 index of refraction, which we append to each. Water, 1.336; 

 glacial acetic acid, 1.38; alcohol, 1.372; vitreous humour, 1,340; 

 sea-water, 1.343; equal parts of glycerine and water, 1.40; pure 

 glycerine, 1.475; oil of turpentine, 1.478; Canada balsam, 

 1.5321.549; bisulphide of carbon, 1.678; oil of annis, 1.811. 

 Alcohol and water, and solutions of various salts in water are 

 also very useful. When a pure article of glycerine is not avail- 

 able, a solution of white sugar may be used with good results. 



Great care must be exercised lest the fluid that is added 

 should change the form or structure of the object. Upon this 

 subject the remarks of Frey are very judicious. He says : " Theory 

 requires that each constituent of the body should be examined 

 in a fluid medium which resembles in respect to quality and 

 quantity, the fluid which saturates the living tissue. Naturally 

 this requirement cannot be completely fulfilled in practice; our 

 aim should be to approach it as nearly as possible. Saliva, 

 vitreous humour, amniotic liquor, serum and diluted albumen 

 are generally recommended as suitable media for the investiga- 

 tion of delicate changeable tissues, and, in certain cases, they 

 accomplish their object in a satisfactory manner. But do not 

 expect them to suffice for every case. Not unfrequently one 

 and the same tissue of different species of animals reacts differ- 

 ently with the same fluid medium, as may be seen with the 

 blood corpuscles. M. Schultze has communicated to us an im- 

 portant and readily proved observation of Landolt's, that ani- 



