1892.] MICROSCOPICAL JOURNAL. 28S 



a photograph or engraving. Hence this medium is scarcely suit- 

 able where color is a leading feature of the object. The solution 

 should be contained in a flat bottle, with clear, polished, parallel 

 sides, as supplied by Baker, and probably other dealers. 



One regrets that the lowest powers (4 in. and under) are so 

 little eniployetl by natural-history observ^ers and amateurs. They 

 aflbrd very useful general views of entire objects, and many 

 striking and beautiful pictures, especially with binocular. This 

 disuse arises from the fact of ordinary microscopes seldom having 

 rackwork sufficiently long to allow of the focusing of very low 

 powers. But a simple adaptation will permit the object to be 

 placed below the stage, so that even a 5-inch objective may be 

 adequately worked on almost any stand. The instrument is like- 

 wise kept steadier, more compact, and less liable to injury than 

 when left out to an extreme length. When there is a " sub- 

 stage" a square plate may be provided with a short tube fitting 

 beneath, so as to fix on the top of it. This may have either 

 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 sufticient 

 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 into holes in some solid part of the 

 stand. The latter plan would be available with simple 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 measui'ements, and assuming a i-inch by Powell 

 and Lealand as a standard, that a 3-inc)i objective is really about 

 a 3f-inch, and a 4-inch some 3^ inches. The so-called "4-inch " 

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



