THE MECHANICAL STAGE 179 



focus an object in the field, with a ^--inch objective in ten or 

 twelve seconds. 



If a perfect mechanical stage cannot be obtained, take no middle 

 course, have ajifm, 'well-made plain one with a smoothly sliding ledge. 

 The stage should be large, and the ledge should glide with perfect 

 ease and without catching when gently pushed from one corner. For 

 this purpose the side-guides should be long, and only the ends of the 

 bar should bear on the stage. The aperture should be as in the 

 mechanical stage, and for the same reason. 



Mr. Nelson suggested a stage of large size, which should have a 

 1^ or 1 J inch aperture bored in it, and then have the intervening 

 brass between it and the front taken away, so that the stage 

 assumes a horse-shoe form. This is thoroughly efficient, and the 

 principle is seen in fig. 134. 



It is a matter of great interest to English microscopists to note 

 that their German collaborateurs in Germany and the leading 

 German makers have not only surrendered to the sub-stage condenser, 

 and even in its achromatic form, but that at length they have also 

 adopted the mechanical stage ; the latest form adopted by Zeiss is 

 figured in the accompanying illustration which shows the complete 

 instrument (fig. 139). We specially call attention to it here, as it has 

 Tun-ell heads, marked H Y, and a rotating stage of 4 inches diameter. 



It must, however, be noted that the usual Continental model 

 adopts a small stage with a %-inch aperture and two fixed spring 

 clips with no sliding ledge ; that is, wanting almost everything 

 required to do good modern work. 



One of the most practical rules for the young microscopist in 

 this relation is, i Have your mounted slide in a fixed position, but 

 never clip it if it can possibly be avoided.' 



In addition to perfect rectangular movements a first-class 

 microscope should have concentric rotary motion to the stage. This 

 is usually effected by rack and pinion, but it is at times desirable 

 to move it with greater rapidity than this admits of. In very well 

 made instruments the pinion engages the rack so lightly that this 

 rapid motion may easily be given to it. In others the pinion can be 

 disengaged and rapid movement effected. 



The centre of rotation of the stage should be closely approximate 

 to coincidence with the optic axis, so that in rotation the object 

 should never be out of the field when a fairly high power is used. 

 Elaborate rectangular centring gear has been used by some makers, 

 and is found in some high-class instruments ; but this is not needful, 

 for all that is really required is to rotate an object without losing it. 

 In fact exact centring w r ould have to be readjusted for every separate 

 objective if it were needed. But any slight departure from the 

 axial centre can be much more readily met by bringing the object 

 into centre by the mechanical stage. 



There are four movements in every microscope ivhich should be 

 graduated : these are (1) the milled head of the fine-adjustment 

 screw ; (2) the stage movements for finders ; (3) the extension draw- 

 tube carrying the eye-piece ; and (4) the rotation of the stage. 

 Divided arcs are imposing, and to the multitude look ' scientific ; ' 



N 2 



