INTRODUCTION. xxyu 



"Lesson supposes that the brilliant hues of the plumage of the Humming-Birds are derived from some 

 elements contained in the blood, and elaborated by the circulation — a theory we do not quite understand, 

 inasmuch as colour is the result of the reflection of some rays and the absorption of others, caused by the 

 arrano-ement of the molecules of any given body. He adds, however, that the texture of the plumes plays 

 the principal part, in consequence of the manner in which the rays of light traverse them, or are reflected 

 by the innumerable facets which a prodigious quantity of barbules or fibres present. All the scaly feathers, 

 he observes, which simulate velvet, the emerald, or the ruby, and which we see on the head and throat of 

 the Epimachi (as the Grand Promerops of New Guinea), the Paradise-Birds, and the Humming-Birds, 

 resemble each other in the uniformity of their formation ; all are composed of cylindrical barbules, bordered 

 with other analogous regular barbules, which, in their turn, support other small ones ; and all of them are 

 hollowed in the centre with a deep furrow, so that when the light, as Audebert first remarked, glides in 

 a vertical direction over the scaly feathers, the result is that all the luminous rays are absorbed in traversing 

 them, and the perception of black is produced. But it is no longer the same when the light is reflected 

 from these feathers, each of which performs the office of a reflector; then it is that the aspect of the 

 emerald, the ruby, &c. varying with the utmost diversity under the incidences of the rays which strike them, 

 is given out by the molecular arrangement of the barbules. It is thus that the gorget of many species takes 

 all the hues of green, and then the brightest and most uniformly golden tints, down to intense velvet-black, 

 or, on the contrary, that of ruby, which darts forth pencils of light, or passes from reddish orange to a 

 crimsoned red-black. 



" It is thus, we think, that the everchanging hues of the gorgets of the Humming-Birds from black to 

 emerald, ruby, crimson, or flame-colour are to be explained." 



In a note just received from Dr. Davy, dated Ambleside, June 10, 1861, that gentlemen says: — *'I have 

 examined with the microscope the feathers of the Humming-Bird, Agi(Bactu cupripennis, you entrusted to 

 me, which is so remarkable for its rich colours as seen in one direction, and only one. The result is merely 

 the following — viz., that those feathers in which this peculiarity is most strongly marked are membranous, 

 terminating in pointed filaments, set on obliquely, so that looking from the head each feather is only partially 

 seen. This result, I apprehend, will help very little to account for the peculiarity in question. Its expla- 

 nation must be sought (must it not ?) in the higher optics." 



"As to the question you ask me about the beautiful play of colours in the Humming-Birds," says 

 Dr. Stevelly, " I have never studied the subject, and I should greatly fear to say anything about it, parti- 

 cularly if what I said were to be looked on as of any authority. 



"There are two optical principles only which I can see to be any way concerned in such an effect. One 

 is the cause of the play of colours in mother-of-pearl, and which Brewster proved to arise from very fine 

 striated rulings, the distance between the parallel lines not being greater than from tlie 10,000th to the 

 100,000th of an inch. Barton, of Birmingham, imitated this by ruling very fine parallel lines on steel dies, 

 and then impressing these on buttons, which showed very beautiful colours when exposed to strong, light. 

 The other optical principle, which I think, however, to be the most likely to produce the eflfect in the case 

 of feathers, is the influence of thin plates. If you know Mr. Gassiot (one of your leading Royal Institution 

 savants), get him to show you some of his copper-plates, on which by an electrotype ])rocess he has had 

 very thin films of lead deposited ; and I think you will see colours fully as beautiful, though not as varied 

 or as variable in different aspects as those of the Humming-Bird." 



