394 THE AMERICAN MONTHLY [Dec. 



With few exceptions only, the working of the tube in its sleeve 

 is much too hard, the makers not having taken into consideration 

 that the preservation of the fine adjustment in a large measure 

 depends on the smoothness of motion in these parts. Often the 

 sleeve is faulty, in some cases being compressed to a slight oval, 

 in others being wider at the lower end than at the upper, where 

 alone the tube is grasped by the slit sleeve, at the lower end 

 allowing an all-around-motion of the tube. Only those in- 

 struments give full satisfaction which possess a tube perfectly 

 cylindrical, and a sleeve of perfect adaptation. 



With gratifying results the author has equipped the tube with 

 a larger ring for manipulation, indeed resembling more a drum, 

 which may be securely held, even in varied positions of the 

 hand. The ring (Fig. 4 and 5), is two inches in diameter, its 

 rim extends down one inch. It has been shown that beginners 

 are much less liable to crush their coverglasses with the tube 

 and drum than with the plain tube, greater comfort being here 

 combined with superior control. With only ordinary skill a i 

 -inch objective may at once be focussed without difficulty. 



Microscope for the Examination of Opaque Objects. — 



Several attempts have been made up to the present time to de- 

 vise apparatus for the illumination of opaque objects examined 

 under the microscope. One of the best known processes is that 

 of Lieberkuhn, which consists in applying around the object- 

 ive an inclined concave mirror, which concentrates the luminous 

 rays in reflecting them upon the preparation. This apparatus 

 cannot be applied unless the frontal distance of the objective is 

 sufficient to permit of the passage of the luminous rays sent ob- 

 liquely. It can, therefore, be employed only for feeble magnifi- 

 cations. Moreover, such oblique illumination is an inconven- 

 ience. 



Mr. Charles Fremont has succeeded in effecting the illumin- 

 ation through the interior of the tube of the microscope and the 

 objective, so that this new method is applicable to even the 

 strongest magnifications. The arrangement adopted as de- 

 scribed to the Academy of Sciences, through Mr. Marey, is as 

 follows: 



The pencil of light n directly projected or reflected by the 

 mirror D, enters the body A, of the microscope tube, through 



