Absorption and the Colours of Thin Plates. 2 1 3 



tremity during the inclination of the plate. In a third specimen 

 the phaenomena were still more varied, and what was a new 

 feature in the results, the colour of the tints was changed exactly 

 as in the phaenomena of absorption. It is very obvious that 

 these results are not produced by the same action which causes 

 the orange colour of the substance, for this action could not 

 vary by the inclination excepting in producing a greater ab- 

 sorption of the more refrangible rays ; but in order to place 

 this beyond a doubt, I detached a film which had none of the 

 colours of thin plates, and which, as I expected, produced' 

 none of the bands above described. In these experiments 

 the nacreous plate was placed in Canada balsam to remove 

 the imperfect smoothness of its surface, but the phasnomena 

 were essentially the same with plates surrounded by air. I 

 now divided the first of the plates above mentioned into two, 

 and having viewed the spectrum through both, I found the 

 principal black band considerably widened, as happens with 

 absorbent media. 



When the light reflected from the nacreous plates is ex- 

 amined in a similar manner, the division of the spectrum into 

 bands is extremely brilliant and beautiful, and the phaeno- 

 mena the same ; but owing to the light having entered the 

 substance to different depths before it was reflected, the spec- 

 trum is by no means complementary to the one seen by trans- 

 mission. 



Satisfactory as these experiments are, I was still desirous 

 of obtaining similar results with perfectly transparent plates ; 

 but after failing in every attempt to combine them, I thought 

 of trying the iridescent films of decomposed glass*. This 

 idea succeeded beyond my most sanguine expectations. I 

 obtained combinations of films which gave me by transmitted 

 light the most rich and splendid colours, surpassing anything 

 that I had previously seen either among the colours of nature 

 or of art. I obtained the deepest and richest blues shading 

 off into the palest, and the finest reds and yellows, with all 

 those intermediate and mixed tints which are seen only in the 

 vegetable kingdom. The reflected tints had quite a different 

 character. They possessed all the brilliancy of metallic re- 

 flexion, like the colours in the Diamond Beetle and other in- 

 sects, and the tints varying within a considerable range were 

 disposed in straight lines and bands, as if the film had formed 

 part of a regularly organized bodyf. 



* For a very fine collection of these films I have been indebted to the 

 kindness of Mrs. Buckland, theMarquis of Northampton, and Mr.Children. 



t The surface of these films is beautifully mammillated, the parts that 

 are curves on one side being concave on the other. 



