ANIMAL STRUCTURES — MODE OF INVESTIGATING. 725 



nitric acid is added to chlorophyle, or chlorine-water to the 

 pigment-cells of the choroid, and so on. 



3. Edge or Border. — This may present peculiarities 

 worthy of notice. Thus, it may be dark and abrupt on the 

 field of the microscope ; so fine as to be scarcely visible ; 

 or it may be smooth, irregular, serrated, beaded, &c. 



4. Size. — The size of the minute bodies, fibres, or tubes, 

 which are found in the various textures of animals, can 

 only be determined with exactitude by actual measure- 

 ment. It will be observed, for the most part, that these 

 minute structures vary in diameter ; so that when their 

 medium size cannot be determined, the variations in size 

 from the smaller to the larger should be stated. Human 

 blood-globules in a state of health have a pretty general 

 medium size, and these may consequently be taken as a 

 standard with advantage, and bodies described as being two, 

 three, or more times larger than this structure ; or all may 

 be measured with a micrometer, as explained at page 51. 



5. Transparency. — This physical property varies greatly 

 in the ultimate elements of numerous textures. Some 

 corpuscles are quite diaphanous ; others are more or less 

 opaque. The opacity may depend upon corrugation or 

 irregularities on the external surface, or upon contents of 

 different kinds. Some bodies are so opaque as to prevent 

 the transmission of the rays of light ; in this case they 

 look black when seen by transmitted light, though white 

 if viewed by reflected light ; others, such as fatty particles 

 and oil-globules, refract the rays of light strongly, and 

 present a peculiarly luminous appearance. 



6. Surface. — Many textures, especially laminated ones, 

 present a different structure on the surface from that 

 which exists below. If, then, in the demonstration, these 

 have not been separated, the focal point must be changed 

 by means of the fine adjustment. In this way the capil- 

 laries in the web of the Frog's foot may be seen to be 

 covered with an epidermic layer, and the cuticle of certain 

 minute -Fungi or Infusoria to possess peculiar markings. 

 Not unfrequently, the fracture of such structures enables 

 us, on examining the broken edge, to distinguish the dif- 

 ference in structure between the surface and the deeper 

 layers of the tissue under examination. 



