86 SUMMARY OF CURRENT RESEARCHES. 



expert microscopist. In fig. 3 are shown the two attachments which 

 effect this change in an ordinary Microscope : — (1) a small prism- 

 box with Society screw to fit the lower end of the Microscope-tnbe ; 

 and (2) an attachment to the upper end of the tube to carry the 

 second eye-piece, with means for adjustment to suit different pupillary 

 distances. The prism-box C contains one compound cemented prism, 

 with transparent silvering on one of the inner faces a b, and a single 

 prism c. The dotted lines show the path of the axial ray, one half of 

 which is transmitted through the compound prism, and the other half 

 reflected into the prism c, and thence to the auxiliary eye-piece. The 

 body of the prism c is extended in the direction of the eye-piece for the 

 purpose of making the optical length of both axial rays alike, so that 

 matched eye-pieces may be used. There are three ways of changing 

 from binocular non-stereoscopic to stereoscopic vision. The first con- 

 sists in covering a portion of the top of the compound prism by a little 

 metal slide. The other two methods depend upon the fact that de- 

 centring the eye-points is equivalent to covering opposite sides of the 

 back of the objective. Hence, if the eye-points are brought about one- 

 eighth of an inch closer together than the observer's pupillary distance, 

 stereoscopic vision is secured ; if they are separated by such an amount, 

 then pseudoscopic vision results. With lew-power objectives and two- 

 inch eye-pieces one may arrange the distance so that, by slightly varying 

 the plane of the eye-points, one may have stereoscopic, non-stereoscopic, 

 or pseudoscopic vision at pleasure, and without moving the eyes far 

 enough to lose any of the field of view. "With high-power objectives, 

 the entire field is seen perfectly only when the instrument is adjusted 

 for non-stereoscopic vision with the eyes in the plane of the eye-points. 



Watson and Sons' Metallurgical Microscope.* — This Microscope 

 (fig. 4) has been constructed exclusively for the examination of metals 

 and minerals, and is of the best quality throughout. The coarse and 

 fine adjustments do not present any novelty. The body is of large 

 diameter, and the draw-tube can be arranged to carry either the Con- 

 tinental or large-sized (1 -27-in.) eye-pieces, as may be preferred. When 

 the draw-tube is closed the body-length is 152 mm. ; when extended, 

 250 mm. The stage has mechanical screws, and in this respect re- 

 sembles the " H " Edinburgh Students' Microscope, made by the same 

 firm. In the centre of the stage is a cylindrical fitting, into which 

 super-stage plates may be fitted and interchanged. The illustration 

 shows a super-stage plate, with le veiling-screws for the purpose of ad- 

 justing the planes of specimens under examination, so as to get them 

 perpendicular to the optical axis. The upper surface of this super-stage 

 is higher than the milled heads controlling the mechanical movements, 

 so that large blocks of metal can be freely moved on the stage. The 

 top-plate measures :»i by t\ in., and can be readily removed and re- 

 placed by a metal-holder, in which blocks of metal can be held at any 

 angle, or rotated. A rackwork of strong, though very smooth and 

 precise construction, is fitted to the stage, and permits it to be moved 

 up and down for focussing after the lighting adjustment has been made. 

 A vertical illuminator, with disc of cover-glass, is provided with the 



* W. Watson & Sons' Catalogue, 1902-3, p. 78. 



