OONSTBUCTION OF THE MICROSCOPE. 75 



its general steadiness, in the admirable finish of its workmanship., and in 

 the variety of movements which may be given both to the object and to 

 the fittings of the secondary or sub-stage. Its disadvantages consist in 

 the want of portability that necessarily arises from the substantial mode 

 of its construction; and in the liability to tremor in the image, when the 

 highest powers are used, through the want of support to the body along 

 its length ( 49). This last consideration has induced Messrs. Boss to 

 adopt the * Jackson-model ' in their more recent Microscopes; the newest 

 iind most complete form of which will be next described. 



72. Ross's Improved Jackson- Zentmayer Microscope. In this admir- 

 able instrument ^Plate v.) the Jackson-model is followed as to general 

 construction, whilst it is improved-on in various important particulars. 

 The 'limb' that supports the principal body with the usual rack-and- 

 pinion slide for coarse adjustment, carries also a second (or focussing) 

 slide at the back of the first, to which a slow up-and-down movement is 

 given by a lever passing through a channel in the limb, which is acted-on 

 by a micronometer- screw with a large milled-head placed in a very acces- 

 sible position. This arrangement renders the fine adjustment quite 

 free from either ' twist ' or ' loss of time,' whilst permitting it to work 

 with sufficient freedom; and has the advantage of not affecting the mag- 

 nifying power by altering the length of the body. Further, if a divided 

 scale (with a vernier) be engraved on the edge of the limb, the thickness 

 of any uncovered object lying on the stage can be measured with great 

 exactness. The rotating stage-plate (graduated at its edge to serve as a 

 Goniometer), is supported upon a firm ring composed of metal of pecu- 

 liar inflexibility; and to this it can be secured in any azimuth by a 

 clamping-screw beneath. Its single traversing platform is moved in 

 rectangular directions by two milled-heads placed on the same axis, that 

 work a combination of screw and pinion (devised by the ingenuity of 

 Mr. Wenham), which is placed above instead of beneath it; and in this 

 device more oblique light (it is' affirmed) can be brought to bear upon the 

 lower surface of the object, than in any other mechanical stage yet con- 

 structed. The stage-ring is not immovably fixed to the limb, but is at- 

 tached to a conical stem, which passes through the tubular pivot of the 

 swinging ' tail-piece ' to be presently described, and is clamped at the 

 back of the instrument by a strong screw and nut. Thus the stage may 

 be made to incline toward either side at any angle, so that a view may 

 be gained of the sides and edges of a solid object, as well as of its front; 

 or it may be removed altogether, and replaced by any other form of ob- 

 ject-support more suitable to the special requirements of the individual 

 Microscopist. The most important novelty, however, consists in the 

 adoption of the (patented) Zentmayer method of giving to the entire 

 illuminating apparatus any desired degree of obliquity. The 'idea' is 

 by no means new; and it was carried- out many years ago by the late Mr. 

 Grubb of Dublin, who fixed beneath the stage a sector or arc-piece of 

 nearly a semi-circle having its centre in the object, upon which the at- 

 tachments of the mirror and condenser were made to slide. But the 

 arrangement devised by Mr. Zentmayer is not only far simpler, but also 

 more effective. It consists in swinging the * tail-piece' which carries the 

 mirrow and the secondary or sub-stage, upon a pivot placed at the back 

 of the stage, the horizontal axis of which is in a line with the point of 

 intersection of the optic axis of the body with the plane of the object on 

 the stage; so that the axis of the condenser shall always pass through 

 that point, whatever may be its inclination to the perpendicular, fiy 



