GLASGOW SOCIETY OF FIELD NATURALISTS. 129 



each object-glass, which is both expensive and not very practicable. 

 "Various methods for obtaining stereoscopic binocular vision under 

 high and the highest powers have been devised, and the best and 

 the one most in use seems to be the one invented by Mr. Stephen- 

 son. In his apparatus the rays on emerging from the object-glass 

 pass through two small prisms, but owing to their considerable 

 length a great deal of light is abstracted. These binocular arrange- 

 ments for bringing into each eye only one-half of the bundle or 

 cone of rays which have passed through the objective, act stereo- 

 scopically. Other methods of enabling the observer to use both 

 eyes have been devised, as for instance Powell and Lealand's 

 patent, but as they reflect identical images into the two eye-pieces 

 their effect is non- or pseudo-stereoscojDic. 



The stand of the compound microscope consists of the support 

 for the body, the focal adjustment, the stage to carry the object, 

 a mirror for illuminating the latter, and which is attached to the 

 so-called tail-piece, and a diaphragm beneath the stage to regulate 

 the quantity of light reflected from the mirror. The most com- 

 plete microscope stands have an additional sub-stage for holding 

 the various pieces of illumination apparatus, which in second or 

 third-class instruments have to be placed into a short tube-fitting 

 attached to the stage. The stand must be heavy, in order to be 

 quite steady and free from tremor. There is a great variety of 

 models of stands, but I shall only mention a few of the principal 

 ones. There is first a di-aw-tube stand, which we find in all old 

 microscopes, and which is still retained by most German and 

 French makers, notably in the excellent microscopes of Hartnacks 

 and Nachet, which I cannot sufficiently recommend on account of 

 their portability, cheapness, excellence of workmanship, and high 

 quality of object-glasses. Then there is the so-called Ross model, 

 in which the body is supported by a rectangular arm ; and there 

 is the Jackson model, the steadiest of all, in which the body is 

 supported along the greatest part of its length. Even the great 

 house of Ross & Co. has, at the suggestion of Mr. Wenham, adopted 

 the Jackson model in 1874, and their new stand I consider to be 

 the handsomest and best of all. 



The coarse adjustment consists in simple microscopes of a draw- 

 tube, and in the better English models of a rack and pinion move- 

 ment by means of large milled heads. The fine adjustment which 

 in Ross' new model is placed behind the body to protect it from 



