4 The Ethereal Hi/jiothesis of Light. [Jan., 



which for convenience I shall call those of Grove and Tyndall;* 

 and in order to guide the students of various branches of physical 

 science in theii' investigations, I propose, first, to select a few pheno- 

 mena for the consideration of the micro-zoologist, chemical and 

 physical experimenter, and mineralogist, and then to point out what 

 appears, in my humble judgment, to be the inference deducible from 

 those phenomena, leaving it to each class of observers to consider 

 the value and accuracy of my investigations, and of the conclusion 

 to which I have been led by them.f 



Fu'st, then, it is familiar to all who have any knowledge of natural 

 history that the brilliant hues of the Lepidoptera, or Butterflies, 

 are due to innumerable minute scales, regularly disposed upon their 

 wings, just as the feathers of birds impart the bright colours to those 

 races. In certain butterflies the wings have an uidescent, or metallic 

 lustre {Lyeoena Adonis, the Chftou Blue) ; in others it is dead and 

 velvety ( Vanessa lo, the Common Peacock) ; whilst in others again, 

 both appearances are intermingled [Polyommatos PhJoeas, the small 

 copper). Now let us inquire to what cause this phenomenon is 

 attributable. 



We will take a specimen of Lyeoena Adonis, the Chfton Blue, 

 of which the blue is quite metallic, or satiny, if I may be allowed to 

 coin the word, and on placing a few of the scales of this insect 

 under the microscope and examining them by transmitted sunlight, 

 that is, by light reflected from the mirror and transmitted through 

 the scales to the eye, we shall find certain of them quite crystalline 

 and transparent (see Plate, Fig. 1) ; others bright orange-red (Fig. 2) ; 

 and others again dusky brown, almost approaching to black. 



Now let us close ofi' the light reflected from the mirror and 

 examine the same scales by incident light, that is, by light concen- 

 trated upon them with the aid of the bull's-eye lens, and we shall 

 find those which by transmitted light appeared translucent and 

 colourless (Fi.G;. 1) to be greenish brown or grey, studded with 

 bright spots (Fig. la) ; those which were orange-red by transmitted 

 Ught (Fig. 2) now appear of a brilhant violet-blue (Fig. 2a), the 

 characteristic blue of the wings themselves ; whilst the dull brown 



* I call the " ethereal " theory Professor Tyiidall's, inasmuch as he has sought 

 to develop it; but those who are interested in its origin and history may refer to 

 tliat author's work, ' Heat as a ]\Iode of Motion.' Prolossor Faraday appears 

 to have given a cautious, or perliaps I should t^ny partial, adhesion to the theory ; 

 and he refers to it once or twice in passiug, iu the IJalctrian Leetiu'e of 1851, 

 whioli I shall quote freely in this article. Sec also Tyndall's ' Faraday as a 

 Discoverer,' p. I'J!). Tjongmans. 



t ]\lr. Clerk INlaxwell, it may be mentioned, considers "light" as a mode of 

 elecfro-mai;netie niotiun. He says, it " consists of allernate and opposite rapidly 

 recurring transverse magnetic disturbances, aec(im]ianii d willi electric clisplacements, 

 the direction of the electric displacement being at rigiit angles to tiu; magnetic 

 disturbance, and both at right angles to the diriction of tiie ray." (Proceedings of 

 the Royal Sneiety, 18U1.) 



