236 THE MICROSCOPE. [December, 



springs or a sliding bar. The former holds a mount securely 

 during rotation, should this be desirable (which is, however, sel- 

 dom the case), but prevents the use of large " growing tanks." 

 If a bar is employed for these, an upper one, or even, on occasion, 

 elastic bands passed over the ends, would be needed to secure 

 mounts if rotated. In using tanks it is almost superfluous to point 

 out that the brass plate must be as much below the optical axis 

 as the whole height of the tank, plus width of the sliding bar, or 

 the top of the tank could not be brought into the field of the 

 microscope. A polarizing prism could often be used in the bot- 

 tom of the substage ; but ordinary-sized Nicols, even when 

 brought up close to the slide, cut ofl^so much of the field as to 

 make it ineflective for display. 



A 5-inch objective needs a prism of a clear inch aperture to 

 avoid " cutting oftV' and this is large and costly. The above ar- 

 rangement is all that can be desired for transparent objects ; but 

 if the substage does not rack far down there may not be sufficient 

 space between it and the bottom of the main stage to admit of 

 the convenient illumination by bull's-eye of opaque objects in 

 deep cells. In such case it is better to attach the supplementary 

 stage to a separate sliding-piece in the substage dovetails, or even 

 by projecting pins dropping jnto holes in some solid part of the 

 stand. The latter plan would be available with sim]ole and only 

 moderate-sized stands. 



Finally, I may remark that this same want of accommodation 

 in the ordinary rackwork has led makers to nominally much 

 underestimate their low powers. A 4-inch objective should mag- 

 nify just four times less than a i-inch ; but I find by numerous 

 careful camera measurements, and assuming a i-inch by Powell 

 and Lealand as a standard, that a 3-inch objective is really about 

 a 2|-inch, and a 4-inch some 3 J inches. The so-called "4-inch " 

 objective is, by the catalogues, the lowest power made ; but 

 Mr. Wray, of Highgate, has recently made, by request, for a 

 friend of mine, a " 5-inch " (true 4j-inch) of superb quality for 

 a "triplet" or " single-system " glass. With E ocular it pre- 

 serves perfect definition, and gives the spirals of tracheae and simi- 

 lar details with full sharpness and absence of color dispersion. 

 Were there any demand, I have no doubt that makers could pro- 

 duce a true 5-inch objective (which they would probably call " 6- 

 inch ") of equal merit. — English Mechanic^ Nov. 18^ i8g2. 



Postal Microscopical Club. — The management of the 

 American Postal Microscopical Club announces that Dr. S. G. 

 vShanks, of Albany, N. Y., has consented to assume the office of 

 secretary of the club, which has unavoidably been vacant for some 

 time past. The Doctor will take charge of the department of 

 slides and their circulation; and all communications pertaining, 

 thereto should hereafter be sent directly to him. 



