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MICROSCOPIC ANALYSIS. SURFACE. xxxi 



4. The colour arising from polarized light is noticed under ANALYTIC CRYSTALS, 

 DICHROISM, and POLARIZATION. 



The colours of objects examined by transmitted light are frequently rendered much darker, 

 and colourless or coloured objects may appear dark or even quite black, from refraction or 

 reflexion of the light out of the field of the microscope. Thus, powdered vermilion appears 

 almost black ; air-bubbles appear black at the margins or entirely black, &c. ; hence the 

 importance of comparing observations made by both reflected and transmitted light, for 

 neglect of this precaution caused the air in the hairs of animals to be mistaken for pigment. 

 Milk-white opacity mostly arises from the presence of numerous molecules, granules, thin 

 layers of liquid or other surfaces which reflect a large quantity of the light incident upon 

 them, as in milk, where the reflecting bodies consist of the globules of fatty matter (butter), 

 in white paper, the two varieties of tubercle, &c. 



3. Structure c/ the Surface. a. When an object is of comparatively large size, the struc- 

 ture of the outer surface is in general easily determined by examining it with reflected light, 



1. e. as an opake object, illuminated by the side-condeuser; but when the objects are small, 

 sufficient light cannot be thrown upon them with ordinary condensers ; recourse must then 

 be had to Dr. Brooke's reflector (p. xi); and this, because it enables us to examine the most 

 minute objects under the conditions in which we can most easily judge of their structure 

 from their appearance. 



b. When this apparatus is not at hand, our judgment must depend upon the appearances 

 presented under the action of transmitted light. And here we meet with a difficult task, in 

 accomplishing which, the following questions are constantly presenting themselves : Do cer- 

 tain spots, hues or other markings visible upon the surface, represent elevations or depres- 

 sions? Are they cavities in the outer portion or layer of the object? Are they foramina or 

 holes ? Are they granules of pigment, or rows of them ? Do the lines represent a true lined 

 structure, or are they optical illusions ? Is the surface smooth and free from markings ? 

 The methods of answering these questions must vary so greatly, according to the nature of 

 the object, its size, &c., that it would be almost impossible to lay them down by rule. The 

 following considerations are, however, of most importance. 



c. In many cases where structural appearances are visible at the surface of an object, their 

 true situation above or beneath the surface may be determined by raising the object-glass 

 above the focus of the surface. On then carefully and gradually depressing the object-glass 

 with the fine movement, the structure first brought into focus is the uppermost. Thus, the 

 inner surface of the under membrane of the elytrum of the stag-beetle (Lucanus cervus), is 

 covered with very minute hairs projecting from the surface (PI. 27. fig. 2). On placing 

 this with the inner side uppermost and adjusting the object-glass as just described, the hairs 

 are distinctly brought into focus before the surface of the membrane. Hence they are 

 situated upon the surface ; whereas, had the surface of the membrane been brought into 

 view before the hairs, it must have been concluded that the latter were situated on a plane 

 below this. It may be stated, that the surface of a membrane is recognized to be in focus 

 by certain irregular granules, molecules or wrinkles, mostly visible upon it. 



d. Frequently, when hairs or filaments project from a surface, their relative position may 

 be easily determined by examining the margin of the object if it be rounded, or the margin 

 of a fold if it be flat and membranous ; as in the case of ciliated bodies, Infusoria, &c. 



e. Cilia upon the surface of an object are sometimes so minute and transparent as to be 

 with difficulty detected ; they can however always be made evident, when present, by the 

 following means : 1 . Drying the object ; they then become much darker from refraction. 



2. Dyeing the object with solution of iodine; drying the object after the addition of the 

 latter solution is sometimes advantageous. 3. Mixing coloured insoluble particles with the 



