56 THE MICROSCOPE AND ITS REVELATIONS. 



to give a support that is free from vibration even when high powers are 

 in use. In the simplest models, such as that of Messrs. Baker (Fig. 40), 

 there is no rack-and-pinion movement for the ' coarse ' adjustment, which 

 can be very easily made by sliding the body through the tube which holds 

 it, provided that this be lined with cloth or velvet; but the \rack move- 

 ment can generally be added at a small cost. A ' fine' adjustment for 

 exact focussing, by means of a micrometer-screw worked by a milled-head, 

 is always provided; and this movement may be given in different ways. 

 In the Continental models, the screw is usually contained within the pil- 

 lar that supports the arm or limb to which the carriage of the body is 

 attached, the milled-head being at its summit (Figs. 40, 45); this answers 

 well if due provision be made to prevent ' twist' of the movable portion 

 (causing lateral displacement of the image), without interfering with its 

 freedom of vertical motion. By many British and American makers, the 

 fine adjustment is made to act on a tube within the ( nose' of the body, 

 into which the objective is screwed; this being raised or lowered, either 

 by a lever contained within the arm, which is acted-on by the milled-head 

 carried by it (as in the original Ross model, Fig. 52), or by a shorter lever 

 at the lower end of the body, to which the milled-headed screw is attached 

 (as in Messrs. Beck's Large Microscope, Plate vn.). This method is 

 subject to two disadvantages: (1) that the focussing tube which carries 

 the objective can scarcely be made to work with the requisite facility, 

 without a liability to ' twist,' which becomes very perceptible after much 

 wear, in the displacement of the image when a high magnifying power is 

 in use; and (2) that by the vertical movement thus given to the focussing 

 tube, the working length of the body, and consequently the magnifying 

 power undergoes change in every adjustment for focus. The plan of fine 

 adjustment which has been adopted from an American model by Messrs. 

 Ross (Plate v.) and is employed in Wale's New Working Microscope 

 (Fig. 44), seems to the Author in every way preferable. Here a lever 

 contained within the limb, and acted-on by a micrometer-screw at its 

 back, gives motion to a long slide, working in dove-tailed grooves, 

 behind the racked slide which carries the body; and this can be made to 

 work very easily, without either ( twist' or 'lost time.' The Stage of 

 Students' Microscopes is often a simple plate of brass, with a couple of 

 springs for holding down the object-slide; but in some models (Figs. 40, 

 45) there is an 'upper stage-plate ' of glass, rotating in the optic axis of 

 the body. Into the aperture of the stage a cylindrical fitting is usually 

 screwed, for the purpose of receiving the Accessories required for giving 

 varied illumination; the most indispensable of these being Diaphragms of 

 different apertures. These should be so fitted that they can be brought 

 up flush with the level of the stage; the limitation of the illuminating 

 pencil for the purpose of obtaining the best definition being much more 

 effectively made by a very small aperture (not exceeding a large pin-hole) 

 close to the under side of the object-slide, than by a wider aperture at 

 some distance beneath it. For the same reason, if a rotating ' dia- 

 phragm-plate ' ( 98) be employed, containing a graduated series of aper- 

 tures, it should be attached to the under side of the Stage itself, and not 

 to the bottom of the cylindrical fitting beneath it. For perfect regula- 

 tion of the light, nothing is so effective as the l Iris-diaphragm ( 98): 

 and this, as Mr. Wale has shown ( 60), may be constructed so cheaply, 

 that its general adoption seems very desirable. The mirror should be 

 double, one of its surfaces plane and the other concave; and it should be 

 so attached (1) that its distance from the stage may be varied sufficiently, 



