THE MICROSCOPE. 
where the object’ is placed. The positive and negative ends of 
the conductor of the electric machine, or the poles of a voltaic 
battery, being put in connection with the handles b b of the 
Fig. 23. 
platinum wire, the spark or current will pass from the point of 
• one of the wires a a to that of the other, being transmitted through 
the object placed between them. 
THE ILLUMINATION OF OBJECTS. 
43. Among the accessories of the microscope, there is none the 
right use. of which is more important than the illuminators. By 
the proper application of these, an infinite variety of beautiful 
effects are produced, and an infinite number of interesting con¬ 
sequences developed, while by their abuse, and by the misconcep¬ 
tion and misinterpretation of their indications, the most fatal 
errors and illusions may arise. 
Let any one, however inexperienced in the manipulations of a 
microscope, applying one hand to the mirror and the other to the 
disc of diaphragms, vary at pleasure the position of the former, 
and turn the latter slowly round its centre, thus shifting the 
direction, and varying the quantity of the light which falls upon 
the object, and he will witness, in looking at the object through 
the instrument, a series of appearances which will soon demon¬ 
strate to him how curious, complicated, and important a part the 
illuminators play in microscopical phenomena. 
44. Objects may be rendered visible in two ways, either by 
light reflected from those parts of their surfaces which are pre¬ 
sented towards the observer, or by light falling on the posterior 
surface, and partially transmitted through them. Opaque bodies 
can be seen only in the former way, but translucent objects may 
be seen in either of these ways. 
A translucent object presents a different appearance, according 
.as it is seen by a front or back light. The leaf of a tree or plant, 
seen by reflected light, appears to have some particular tint of 
42 
