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. Not 

 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 



