MICROMETER MICROSCOPIC APPARATUS. 615 



firmly attached to the axis, as is also the double mirror, /. The triangular bar, 

 g, has a rack on its posterior part, which is worked by a pinion, the milleclneads 

 of which are seen at h, h. The body, z, screws firmly into the arm,J, the object 

 glasses contained in the boxes, , Z, screw into the body at m ; the eye-piece, n, 

 or o, slides into the other end of the body. The mirror is plane on one side, and 

 concave on the other, and is fitted with a universal movement, so as to be in- 

 clined in any desired position. The milled heads, /, A, by being revolved, raise 

 or lower the body, i, and constitute the coarse adjustment ; the fine adjustment 

 is effected by turning the milled head, p. The object to be examined is placed 

 on the stage, rf, and retained in the required position by the sliding piece, e. The 

 quantity of light admitted through the instrument may be modified by the dia- 

 phragm, r, which consists of a plate of brass with four apertures of different dia- 

 meters, made to revolve on a central pin or axis fixed to the bottom of the stage. 

 In addition to the four holes mentioned as needed to admit the requisite amount 

 of light, the diaphragm is furnished with a fifth hole, into which the mounted 

 Xicol's prism, s, screws, forming the polariser ; the analyser, t, is screwed into 

 the upper part of the adapter of either, k or Z, previous to its being attached to 

 the body, i. The polariser, s, is mounted in a double tube, so as to be capable 

 of being revolved by turning the large milled head at the bottom. The con- 

 densing lens, u, is for illuminating opaque objects, and fits by a pin, c, into the 

 hole at the corner of the stage ; it is so fitted that it can be used in any required 

 position or angle. The forceps, i;, are for lifting small objects. The stage for- 

 ceps, 10, fit the hole in the corner of the stage, </, and serve to hold any opaque 

 or transparent object while being examined. The live, or animalcule box, x, 

 is for retaining insects, &c. in the field of view, and for the examination of a 

 drop of water containing animalcule. The adapter, ^/, may have the object 

 glass, k or Z, screwed into it, and be itself screwed into the arm, J, in the place 

 of the body, t, when it is desired to form a Simple Microscope. 



Among the objects furnished with the Microscope will be found a plate of 

 selenite, which, if laid under many animal and vegetable structures while being 

 examined by polarised light, will cause them to assume the most gorgeous colours, 

 although without this addition they may be perfectly colourless. 



The achromatic object glasses, , consist of three plano-convex lenses, and 

 the set Z, of two. All of each set are used together when the highest magnify- 

 ing power is required, but where a lower magnifying power is needed, one lens 

 only may be used ; care must be taken not to mix the lenses of one set with those 

 of the other.* Editor.'] 



1196. Micrometer. In measuring the size of objects, a micrometer (/unc^os, 

 small, and psrpoy, a measure) is employed. The stage micrometer consists of a 

 piece of glass, ruled with fine lines by means of a diamond point, at some known 

 distance apart, such as the T 5 or T ^g 5 of an inch. This is laid on the object- 

 glass plate, and the substance to be measured is placed on it. It is necessary 

 that the object and the lines should be seen at the same time, which cannot be 

 always accomplished. The eye-piece micrometer consists of an eye-piece 

 having a glass divided bylines, varying from ^ 5 to T J 5 of an inch apart, placed 

 either in the focus of the eye-lens, or below the field-lens. Other kinds of 

 micrometers are also employed, such as the cobweb micrometer, where, by 

 the motion of a screw, fine wires or cobwebs are made to separate from each 

 other. 



1197. Microscopic Apparatus. In delineating minute structures, it is use- 

 * These two microscopes are manufactured by the publishers of this volume. 



