THE MICROSCOPE. 
Fig. 19. 
slowly moved right and left, and by turning the other it may he 
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 
Fig. 20. 
with two moveable 
stages superposed is 
drawn in fig. 20, 
where a a a a is the 
fixed stage, and b b 
b b, c c c c the two 
stages which move 
in the grooves n n 
and m m , the one 
bbbb 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 
38 
