MICROSCOPIC ANALYSIS. COLOUR . xxxv 



or from the presence of pigment ; and when the colour of an object obeys the above law, 

 it may be predicted that structural irregularities sufficient to account for its production 

 ■will be found if properly sought for. Moreover these colours are not the same by 

 reflected and refracted light, and they vanish under very high powers. They may be 

 studied in the species of Pleurosigma ; and those observers whose microscopes do not 

 magnify sufficiently, or whose object-glasses have not sufficient angular aperture to admit 

 of the detection of the markings upon some of the Diatomacese or other bodies of similar 

 structure, may be sure that they are present when these phenomena have been observed. 

 AVe were thus led to search for them upon the valves of Melusira varians and Borreri, 

 species of Nilzschia, Sec, where they had not been previously detected ; and there they 

 are present. Again, the colours of the dried valves of the Diatoniaceae, many of which 

 have a brown tinge, have been supposed to depend upon the presence of the peroxide 

 of iron ; but as this colour vanishes when the valves are immersed in oil of turpentine, 

 independently of the fact that the valves do not present the same brown colour by 

 reflected and transmitted light, and by direct and oblique light, which we have stated to 

 be characteristic of the presence of colouring-matter, the colour cannot arise from this 

 cause. 



An example of iridescent colour arising from the presence of fibres, is found in the 

 tapetum. Certain cases, referable to this head, require special notice. Thus it sometimes 

 becomes a question whether a very minute red spot, visible in an Infusorium, Alga, &c, 

 is the optical expression of a minute vacuole, or a little depression filled with water, ah', 

 or other fluid of less highly refractive power than the substance of which the organism 

 consists, or whether it arises from the presence of pigment. The point is easily decided : 

 a practised eye will recognize the transparency of the colour where not arising from 

 pigment, and its granular appearance where the pigment is present. If the substance of 

 the object be soft, compression will frequently destroy the appearance when pigment is 

 absent. Drying the object and then immersing it in oil of turpentine or other highly 

 refractive liquid will do the same, whilst pigment will become even more distinct if present. 

 Moreover, on altering the focus of the object-glass, the colour will be found to change, 

 when not arising from pigment. 



The colours of thin plates are so rare in microscopic objects, that we must refer to works 

 upon optics for an account of them. They occur in the crystals found upon the surface of 

 the scales of various fishes, the eggs and wings of insects, &c. 



(4.) The colour arising from polarized light is noticed under Analytic crystals, 

 Dicheoism, 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 reflection 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 

 globides of fatty matter (butter), — white paper, tubercle, &c. 



", Structure of the Surface. — a. When an object is of comparatively large size, the 



