THE MICROSCOPE. 



abandoned; and the same remark is applicable to the attempts 

 at constructing fluid lenses, and to the projects for giving to 

 glass other than spherical surfaces none of which have come 

 into extensive use. 



By the term simple microscope is meant one in which the 

 object is viewed directly through a lens or combination of 

 lenses, just as we have supposed an arrow or an insect to be 

 viewed through a glass held in the hand. When, however, the 

 magnifying power of the glass is considerable, in other words, 

 when its focal length is very short, and its proper distance from 

 its object of consequence equally short, it requires to be placed 

 at that proper distance with great precision: it cannot, there- 

 fore, be held with sufficient accuracy and steadiness by the un- 

 assisted hand, but must be mounted in a frame having a rack 

 or screw to move it towards or from another frame or stage 

 which holds the object. It is then called a microscope, and it 

 is furnished, according to circumstances, with lenses and mir- 

 rors to collect and reflect the light upon the object, and with 

 other conveniences which will now be described. 



One of the best forms of a stand for a simple microscope is 

 shown in Fig. 10, where A is a brass pillar screwed to a tripod 

 base; B is a broad stage for the objects, secured to the stem by 

 screws, whose milled heads are at 0. By means of the large 

 milled head D, a triangular bar, having a rack, is elevated out 

 of the stem A, carrying the lens-holder E, which has a hori- 

 zontal movement in one direction, by means of .a rack worked 

 by the milled head F, and in the other direction by turning on 

 a circular pin. A concave mirror G reflects the light upwards 

 through the hole in the stage, and a lens may be attached to 

 the stage for the purpose of throwing light on an opaque ob- 

 ject, in the same way that the forceps H for holding such ob- 

 jects is attached. This microscope is peculiarly adapted, by its 

 broad stage and its general steadiness, for dissecting; and it is 

 rendered more convenient for this purpose by placing it be- 

 tween two inclined planes of mahogany, which support the 

 arms and elevate the wrists to the level of the stage. This ap- 

 paratus is called the dissecting rest. When dissecting is not a 

 primary object, a joint may be made at the lower end of the 

 stem A, to allow the whole to take an inclined position; and 



