THE MICROSCOPE. 



Fig. 19. 



slowly moved right and left, and by turning the other it may be 



moved backwards and forwards, 

 and, in fine, by turning both 

 at the same time it may be 

 moved diagonally in any inter- 

 mediate direction, according to 

 the relative rate at which the 

 one and the other milled head 

 is turned. Sometimes the two 

 milled heads are on the right 

 side of the stage, so that they 

 can be turned either separately or together by the right hand, 

 and sometimes they are placed at opposite sides, so as to engage 

 both hands. 



38. It is generally found convenient to have an easy means of 

 turning the object round its centre, so as to present it to the light 

 in all possible positions, without displacing it from the centre of 

 the field. This is accomplished by inserting in the upper plate 

 of the stage a metallic disc of somewhat greater diameter than 

 the central aperture of the stage, which is so fixed as to be turned 

 smoothly round its centre. It is upon this disc that the slide is 

 placed and held by the springs which are attached to the disc so 

 as to turn with it. This disc is sometimes graduated in 360, so 

 that the observer can turn the object through any desired angle, 

 a power which will be found very convenient in certain classes 

 of observations. 



The arrangement consisting of a fixed with two moveable 



stages superposed is 



Fi e- 20 - drawn in fig. 20, 



where a a a a is the 

 fixed stage, and b b 

 b by c c c c the two 

 stages which move 

 in the grooves n n 

 and m m, the one 

 b b b b directed right 

 and left, and the 

 other c c c c back- 

 wards and for- 

 wards. The grooves 

 in which the upper 

 stage c c c c moves 



are formed in the lower stage bbbb, and those in which the 

 latter moves are formed in the fixed stage a a a a. The one 

 stage is moved by turning the milled heads s s fixed upon the 



