PHENOMENA OF ABSORPTION, AND THE COLOURS OF THIN PLATES. 243 
exactly similar to those of coloured media. In one specimen I obtained a dark and 
distinct band in the orange space at D, with another faint band in the red. These 
bands were parallel to the fixed line D at a vertical incidence, but by inclining the 
plate the bands moved towards the green space, and became inclined to the line D. 
In a recent specimen I obtained the darkest band in the green space, with other lesser 
bands of unequal size and breadth in the other spaces, all of which moved along the 
spectrum, while new ones advanced from the red extremity during the inclination of 
the plate. In a third specimen the phenomena were still more varied, and what was 
a new feature in the results, the colour of the tints was changed exactly as in the 
phenomena 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 absorption 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 
phenomena 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 examined in a similar manner, 
the division of the spectrum into bands is extremely brilliant and beautiful, and the 
phenomena the same ; but owing to the light having entered the substance to dif- 
ferent depths before it was reflected, the spectrum is by no means complementary to 
the one seen by transmission. 
Satisfactory as these experiments are, I was still desirous of obtaining similar re- 
sults 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 suc- 
ceeded 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 pos- 
sessed all the brilliancy of metallic reflexion, like the colours in the Diamond Beetle 
and other insects, and the tints varying within a considerable range were disposed in 
straight lines and bands, as if the film bad formed part of a regularly organized body -f~. 
* For a very fine collection of these films I have been indebted to the kindness of Mrs. Buckland, the 
Marquis of Northampton, and Mr. Children. 
f The surface of these films is beautifully mammillated, the parts that are curves on one side being concave 
on the other. 
