MICROSCOPIC INSTRUMENTS. 363 



accompany more expensive instruments, which we pro- 

 ceed to describe. 



A Moveabh Stage, D, composed of a si id ing-plate, 

 which is made to move over another plate by means of 

 two screws placed diagonally, a curved spring keeping 

 the plate in contact with the screws. By this contrivance 

 an object, when laid on the stage, is capable of being 

 moved in any horizontal direction, so that every part of 

 it can be successively brought into view ; the lower plate 

 has three pieces of brass projecting from its circumference, 

 to fit over the edge of the stage-plate. 



The Stage Micrometer, M, for measuring the size of 

 small objects, consists of a glass about a quarter of an 

 inch in diameter, set in a brass frame about two and a 

 half inches long, and one third wide. On the surface of 

 the glass is ruled with the point of a fine diamond, a scale 

 occupying one twenty-fifth part of an inch, within which 

 are accurately divided one hundred equal parts; each 

 division is therefore one twenty -five hundredth of an inch 

 apart. These minute divisions are clearly discerned, and 

 have each a visible length, even under the low powers 

 of the microscope ; and by laying a small object, as a 

 fibre of silk, or a hair, across this scale, its dimensions 

 can be accurately determined, as the object and the lines 

 on the glass can be seen at one and the same time ; if the 

 breadth of the object occupies twenty-five divisions of the 

 scale, its breadth is one hundredth of an inch ; if it covers 

 ten only, its breadth is one two-hundred-and -fiftieth of an 

 inch. 



The Camera Lmida is a valuable addition to a micro- 

 scope ; it consists of a four-sided prism of glass, set in a 

 brass frame or case, and by means of a short tube, or 

 spring clip, applied to the tube of either of the eye- 

 pieces, so that the prism may be directly in front of the 

 eye-glass. To use conveniently, the microscope should 

 be laid in a horizontal position, and supported at a proper 

 distance from the table for convenient use, say from one 

 to two feet, and a sheet of white paper laid on the table 

 beneath the prism ; if now the light be reflected through 

 the compound body, an eye placed over the small hole in 

 the frame of the prism, with the sight directed toward the 

 paper, will see the image of any object on the stage, if in 

 the foous, on the sheet of white paper. Should the whole 



