THE MECHANICAL STAGE. 45 



useful addition to the more perfected forms of instru- 

 ments. This tailpiece, represented in sectional elevation 

 fig. 27, consists of s, the limb carrying the body, with 

 coarse and fine adjustments ; A, the stem carrying the 

 sub-stage, B, and mirror. A is attached to s by the sleeve 

 or socket I, clamped by the nut j, and on I A may be 

 swung sideways in either direction to the right or left, 

 either below or above the stage, the axis of revolution 

 of which is the line x T ; that is, a line in the plane of 

 the object to be viewed on the stage c, intersected by 

 the optical axis of the instrument ; that is, the line N o, 

 passing through the centre of the body and the object- 

 glass of the microscope. The stage c is also attached 

 to s by the pin C 1 , terminated by the screen C 2 , which 

 pin passes through the centre of the socket I, and 

 moves therein so that the stage C may readily turn in 

 either direction in conjunction with or independent of 

 A, the axis of its revolution being also the line x T. 

 By this kind of arrangement the stage C and the stem 

 A can be set at any angle to the axis of the microscope, 

 either below or above x Y, intersecting the plane of the 

 object to be viewed, and relatively to each other, and 

 when so set the stage C can be clamped at the desired 

 angle by the nut D on the screw C 2 acting on s and the 

 collar K. 



A mechanical stage is one consisting of two or more 

 plates, the rectangular motions of which are obtained 

 by rack and pinion. It is considered a necessary 

 appendage to the more finished instruments. The 

 cheaper kinds are provided with a simple form of 

 sliding plate, the lower part of which is a raised edge 

 for the objects to rest against. It is often found con- 

 venient to have a means of watching growing and 

 other processes, which are either promoted or assisted 

 by maintaining the object for a time at a certain tem- 

 perature. For this purpose many forms of apparatus 

 have been contrived. A very inexpensive form is that 

 of Mr. Bartley's, fig. 28. 



The vessel E, three parts filled with water, and sup- 

 ported on a ring-stand, is kept at any temperature by 

 the spirit lamp c, a syphon-tube d conveys the Jbot 



