XXX INTRODUCTION. 



or granular powder ; as a white animal hair first macerated in solution of ferrocyanide of 

 potassium and then in solution of perchloride of u'on. Chemical means will alone distin- 

 guish this cause of colour from the first, by removing the colouring matter from the colour- 

 less basis. 



3. The colours of many objects vary according to the direction of the light transmitted 

 through them, or are only visible by oblique light, and the colom's are different by direct 

 and oblique light. These arise from decomposition of white light by either interference or 

 refraction. For the sake of brevity, these may be designated colours from iridescence, be- 

 cause they mostly exhibit the brilliancy and transparency of the colours of the rainbow. The 

 interference or refi'action upon which they depend is ordinarily produced by u'regularities of 

 structure, frequently depressions or grooves, and sometimes cavities containing air, &c. Ob- 

 jects exhibiting these colom's, which are most brilhant by veiy oblique hght and under low 

 powers, when examined with a moderately high power by transmitted du'ect or but shghtly 

 oblique light, frequently appear more dull and less brilliant, often dark or black in parts ; 

 and when immersed in oil of turpentine or some liquid approaching in refractive power the 

 substance of which they are composed, so that their irregularities become filled with it, the 

 colours vanish. Hence colour, when arising from iridescence, can readity be distinguished 

 from that arising from general absorption 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 

 colom's are not the same by reflected and refracted light, and they vanish under very high 

 powers. They may be studied in the species of Gyrosigma ; 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 phaenomena have 

 been observed. We were thus led to search for them upon the valves of Melosira varians, 

 and Borreri, species of Nitzschia, &c., where they had not been previously detected, and there 

 they are present. Again, the colours of the dried valves of the Diatomaceae, many of which have 

 a bro\ATa tinge, have been supposed to depend upon the presence of the peroxide of u-on ; 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 pre- 

 sence 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 tape- 

 tum. Certain cases, referable to this head, require special notice. Thus it sometimes be- 

 comes 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, air, 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 tm-pentine 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 colom-s of thin plates may be mentioned here ; but they 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 under surface of the scales of various fishes. 



