130 TKANSACTIONS OF THE 



injury, consists of a fine micrometer screw, having in good micro- 

 scopes from 100 to 200 turns to the inch. This screw acts by- 

 means of a lever upon the so-called nose-piece in the lower end of 

 the body into which the object-glass screws, and which is pressed 

 downwards by means of a spring. I must not forget to state that 

 the Royal Microscopical Society in London has introduced a stand- 

 ard gauge of screw, with which the nose-pieces of all good English 

 microscopes and all good English object-glasses are now-a-days 

 provided. By means of one of the best fine adjustments we can 

 easily focus through a distance of only the one five-thousandth part 

 of an inch, and it serves at the same time to measure i-oughly the 

 height or depth of small objects. 



The stage may either be plain or mechanical, and a good plain 

 stage is infinitely preferable to a bad mechanical one. The stage 

 should be large, have a large opening in the centre, and ought to 

 be as thin as possible, to allow of the most oblique illumination. 

 There ax^e various contrivances in use to assist in moving the slide, 

 but the ordinary mechanical stage consists of rectangular rack and 

 pinion and screw movements. In addition to these rectangular 

 movements there is a circular movement which is seldom mechani- 

 cal. The latest perfection of the stage is the concentric rotating 

 movement, which allows of an object being kept in the field of 

 view when the stage is rotated. In order to obtain this concentric 

 rotating movement, the rectangular movements are placed on the 

 rotating movement, and not the rotating stage on the rectangular 

 movement, as in the ordinary mechanical stage. To some simple 

 instrument without rectangular movements a concentric stage is 

 sometimes added, which is generally made of glass. In the larger 

 instruments the concentric rotating stage is graduated into 360°. 



The diaphragm is placed under the stage or in the sub-stage. 

 It consists in its simplest form of a disc of metal having various 

 sized apertures for admitting more or less light on the object. 

 The graduating diaphragm is a superior contrivance for gradually 

 reducing or increasing the aperture of the diaphragm, which is of 

 great importance for obtaining the best effects. 



The sub-stage is only added to first-class and to some of the 

 second-class microscopes. It has generally a rotating movement by 

 means of a wheel and pinion and two centring screws. A. rack 

 and pinion raises or lowers the sub-stage, which is also generally 

 graduated into 360". Ordinary microscopes have, as mentioned 



