6 MANUAL OF HISTOLOGY. 



right, so that the pencil of light may be thrown across the 

 object. By this means, little inequalities of the surface which 

 would be invisible under direct light are clearly demonstrated. 

 The poorer lenses, however, are those which necessitate oblique 

 light. When reference is made to the definition of the lens, 

 direct light is intended. 



Arrangement of tlie object. When the object is to be ex- 

 amined, it should be placed upon the glass slide, which is usu- 

 ally one by three inches in superficial measurement, and as 

 thin as is compatible with the usages to which it is put in 

 ordinary microscopic work. The glass should be white in color, 

 and free from any imperfections that can be detected by the eye. 

 Usually a drop or two of water, a drop of glycerine, or a drop 

 of water and glycerine in equal parts, is placed upon the- slide. 



The object is then immersed in the liquid. It takes some 

 little time for the fluid to permeate the specimen, so that it is 

 ready for study. When pure glycerine is used fully ten minutes 

 will generally elapse before the specimen is transparent. A 

 covering glass is then cautiously let fall upon the liquid, care 

 being taken that no bubble of air enters. The cover is then 

 pressed down. In such cases, when the object is studied with 

 high powers, the cover will often slowly rise and separate itself 

 from the slide, so that the forceps or the finger may be neces- 

 sary to press it back. This inconvenience is obviated by paint- 

 ing a little Canada balsam or cement around the edge of the 

 cover so as to hold it down. 



The kind of a lens to be used. For the first examination a 

 low objective should be used, with a medium, not short, eye- 

 piece. The tube should then be carried down until the object 

 comes within the focus. Low powers should always be used 

 at first, because they give a good idea of the object in its gen- 

 eral features. 



Then the tube may be withdrawn, and a higher power sub- 

 stituted, and so on, until the specimen has been studied in all 

 its details. A convenient accessory is now made by most of 

 the instrument makers; it is a "nose-piece" a brass attach- 

 ment which is screwed into the end of the tube, and carries two 

 or more lenses. 1 



1 The double angular nose-piece made by Schrauer, 46 Nassau Street, costs 

 the triple, $20 ; all of the microscope makers are now prepared to furnish them. 



