ZOOLOGY AND BOTANY, MICROSCOPV, ETC. 323 



has advantages in connexion with ilhimination. The illumination in 

 aqueous media is less satisfactory. 



"When the glass discs are not in use, it is best to keep them in 

 absolute alcohol in a glass-stoppered bottle. They should not be 

 allowed to become dry with mounting fluid in the capillary orifice, 

 otherwise they will be very troublesome to clean out. 



De>itaining of Nemas or other Small Objects in the Differentiator. — 

 In handling a mass of small organisms by the differentiator method, 

 there is sometimes considerable difficulty in securing satisfactory 

 destaining. There is little difficulty in getting a mass of organisms 

 thoroughly impregnated with the stain, no matter how varied they 

 may be in species and in size ; it is simply a matter of time. The 

 trouble comes in destaining. If the destaining process is carried on 

 until the largest of the objects, or the most impenetrable ones, are suf- 

 ficiently destained, it will generally happen that smaller specimens, or 

 those more easily penetrated, are deprived of too much of their colour. 

 It is therefore a matter requiring considerable experience and judg- 

 ment to successfully destain such a miscellaneous collection. The 

 difficulty is considerably increased by the fact that when enclosed in 

 the differentiator tube, the specimens are not very easy to examine 

 critically l)y any ordinary method. If the differentiator be held toward 

 a strong light, the organisms may be examined by the aid of an ordinary 

 pocket lens, but not very critically. The most satisfactory piece of 

 apparatus for this work is what is sometimes known as the chemical 

 microscope, in which the objective is below the stage and the light that 

 passes through it from above is reflected by a prism placed below so as 

 to pass obliquely upward through a barrel carrying an eye-piece. If 

 the differentiator tube containing the destained nemas is laid on a glass 

 stage over the objective of such an inverted microscope, and a little 

 water, or still better, cedar oil, be placed between the differentiator tube 

 and the glass stage, it will be found that the nemas or other objects 

 will sink to the bottom of the fluid in the differentiator tube so as to 

 come as near as possible to the ol>jective of the microscope. If the 

 glass stage is thin, there is no difficulty in using a one-half to two-thirds 

 inch objective. In this way, the nemas may be examined more critically 

 with regard to the extent of the destaiuiug. 



If it is desired to use a lens of higher power, it is sometimes possible 

 to do so by resorting to another method. Place a cover-glass on a 

 horizontal surface, and on the cover-glass a good-sized drop of cedar 

 oil. Lay the differentiator tube into this drop of cedar oil in such a 

 way that the nemas come opposite the cover-glass. It will now be 

 found that the cover-glass will adhere to the differentiator by capil- 

 larity, so long as the differentiator is held in a horizontal position. If 

 the chemical microscope stage has a large aperture, it will be possible 

 to lay the differentiator across the stage, cover-glass downward. In this 

 way, if the differentiator tubing is thin, it will be possible to use even 

 <iuarter-inch objectives of long focus. 



^Yhere considerable work is done with differentiators, a chemical 

 microscope used in this way is a valuable accessory. 



Compressor ium for Chromosomes. — When chromosomes or other 



