296 Intelligence and Miscellaneous Articles. 



completely vanished, while the white presented the usual ghastly 

 appearance. A blue book-cover, treated in the same manner, gave 

 more reflection than did the blue coating. 



Experiments with the polariscope were at first inconclusive, from 

 the fact that though the light from the blue coating was largely 

 polarized, so, to some extent, was also that irregularly reflected 

 from the charcoal, and it was found necessary to cover the block with 

 a thiii layer of carbon from a gas-flame. The repetition of the test 

 then showed that the proportion of light polarized by the layer 

 of carbon, at the given angle, was almost nothing ; that by the 

 thick white coating, small, while on the blue the phenomenon 

 was almost complete. What light here was not polarized was 

 evidently reflected from the larger particles mixed with the fine; 

 for the analyzer, while it did not totally extinguish the light, yet 

 excluded nearly all appearance of blueness. 



In order to determine the character of the transmitted light, a 

 microscope covering-glass was inlaid in the charcoal and the 

 oxidation so executed that the glass was in the centre of a small 

 area, all of which was blue. On removing the glass, the light which 

 passed through proved to be of the expected yellow, though less 

 brilliant than anticipated. The colour might be seen either by 

 transmitting the direct light of the sun, or by placing the glass at 

 such an angle that total reflection was produced, and thus in the 

 passage of the rays through the layer to the glass and out through 

 the layer to the eye the blue was principally lost and only the 

 mixture of longer rays appeared. Viewed through a microscope, 

 the result was the same. I have since, however, improved upon 

 this plan by the more convenient method of covering with carbon 

 a piece of ordinary window-glass, three inches by two, and then 

 projecting the oxide upon the opposite surface of the plate. There 

 is thus no difficulty in distinguishing a very slight amount of 

 colour in the coating ; and for transmitted light any portion of the 

 carbon may be easily removed. 



This case, a type of all charcoal coatings which shade off to 

 blue in thin layers, appears thus parallel to that of the sky-colour ; 

 and the theory which is accepted for the one will also satisfactorily 

 explain the other. 



To account for the cadmium-green we have only to note that, 

 if the substance upon which we are experimenting have the power 

 of absorbing the shorter rays of the spectrum, the reflected light 

 would from a heavy coating be yellowish or reddish, the particular 

 shade depending upon the amount of absorption of violet and blue, 

 and the formation of a layer as thin and of particles as fine 

 as before should result in giving us the colour of the shortest 

 rays which the substance is capable of reflecting, viz. in this case, 

 green. The coating of cadmium has exactly this appearance, and 

 shows the effect of the gradual transmission of red by shading 

 from the original colour (dark red) through yellow into a fine 

 green. As before, the light reflected from the thin layers is highly 

 polarized, and the rays which pass through form a deep, dark red. 



