THE MICROSCOPE. 
the interposition of smaller diaphragms, until the best effect is 
produced. The observer will acquire by practice a facility in 
making these adjustments, so as to produce the desired result. 
On the other hand, if the object be very imperfectly trans¬ 
lucent, the light thrown upon it must be rendered as intense as 
possible by the contrary arrangements. 
46. Different parts of the same object will generally have 
different degrees of translucency, and it will often happen that a 
light which would drown the more transparent parts will be no 
more than sufficient to display the more opaque parts. In such 
cases the observer will have to vary the light according as his 
attention is directed to one part or the other. 
It must not be inferred that the darker parts are in this case 
really darker than those which are more transparent. The 
lesser degree of translucency more frequently arises from the 
different thickness of different parts of the object, the thicker 
parts absorbing more light, and therefore appearing of a darker 
tint than the thinner. If the varying transparency arise from this 
cause, the apparent lights and shadows or tints of colour must be 
taken as mere indications of the inequalities of thickness of a 
body of which the real colour is uniform. 
The difficulty which an observer encounters in ascertaining the 
real form of an object, and the accidents of its surface when seen 
in a microscope by a back light, is partly owing to the fact that 
the eye is habituated to view objects almost exclusively by front 
lights, and the impressions produced of their forms are always 
deductions of which we are rendered unconscious by habit, by 
which the characters of these surfaces are inferred from the lights 
and shadows which are impressed on the organ of vision. Hot 
having the same habit of seeing objects by a back light we cannot 
so easily make similar deductions, and we are apt to judge of the 
objects as if in fact they were illuminated with a front light. 
The judgment is also more or less perplexed, and deceived by 
the fact that microscopic objects are as it were placed before the 
eye in an unnatural state of proximity, which give them a visual 
character totally different from that which objects have, viewed 
in the usual way with the naked eye. 
It must be evident, therefore, how much attention and address 
on the part of the observer are indispensable to enable him to 
disentangle their physical causes from such complicated effects, 
and to give their appearances a right interpretation. 
47. If an object, of which the surface is marked by numerous 
inequalities and asperities, be illuminated by a light which falls 
perpendicularly upon it, or which is scattered indifferently in all 
directions, an observer placed directly over it will be in general 
44 
