388 THE MICROSCOPE. 



according to the mode of illumination ; that is, as the light is reflected - 

 from or transmitted through their substance, as in the case of certain 

 scales of insects, feathers of birds, &c. Colour is often produced, 

 modified, or lost, by re-agents ; as when iodine comes in contact with 

 starch-granules, when 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 j 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 measurement. 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 con- 

 sequently be taken as a standard with advantage, and bodies may be 

 described as being two, three, or more times larger than this structure. 



5. Transparency. This physical property varies greatly in the ulti- 

 mate elements of numerous textures. Some corpuscles are quite dia- 

 phanous ; others are more or less opaque. The opacity may depend 

 upon corrugation or irregularities on the external surface, or upon con- 

 tents 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 peculiar luminous appearance. 



6. Surface. Many textures, especially laminated ones, present a dif- 

 ferent 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 

 capillaries 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 

 difference in structure between the surface and the deeper layers of the 

 tissue under examination. 



7. Contents. The contents of those structures which consist of 

 envelopes, as cells, or of various kinds of tubes, are very important. 

 These may consist of included cells or nuclei, granules of different kinds, 



