1869.] The Ethereal Hypothesis of Light. 5 



scales (Fig. 3) are tlie least changed of any, being ratlier lighter 

 (Fig. 3a) and presenting a steel-like surface. 



The cause of these changes is quite obvious. 



In the scale which was translucent and colourless by trans- 

 mitted, but brownish-grey under incident light, a portion of the ray 

 (as I shall for the present call it) is reflected back to the eye in the 

 latter condition ; that is to say, whilst it passes unimpeded through 

 the scale in the first instance, it is arrested in the second, being 

 partly absorbed and partly reflected. In the scale which appeared 

 dififerently coloured under both aspects, namely, red by transmitted, 

 and blue under incident light, the ray was arrested m both in- 

 stances, the same part, namely, the orange-red passing through, and 

 the other (the blue) being reflected. In the first instance, we saw 

 the ray which had passed ; in the second, that which was unable to 

 pass, but which was reflected, (It must always be borne in mind 

 that for the present I speak popularly, for we shaU presently con- 

 sider what " ray " and " portion of ray " really mean.) In the 

 third example (3 and 3a) there is secreted in the scale itself a sub- 

 stance which has the power of arresting certain rays when the light 

 enters from above (incident) less than when it strikes upwards from 

 below (transmitted). Because, in the first instance, the scale surface 

 arrests and reflects a portion of the light before even it enters the scale, 

 as exemplified also in Fig. 2a, or perhaps more characteristically still 

 in another species, Lycoena Alexis, where the scale is pale brown (also 

 caused by pigment) by transmitted, and pearly opal (Fig. 5) under 

 incident light. In the "Admiral" (Vanessa Atalanta) the efiect is 

 as nearly as possible the same under both conditions, the colour 

 being due to pigment. Fig. 4.* 



Deferring for a time the consideration of the bearing of these 

 phenomena upon the ethereal hypothesis, I will now direct the 

 reader's attention to the results of a few of the elaborate and inte- 

 resting experiments of the late Professor Faraday, connected with 

 "the relations of gold and other metals to light." These were 

 fully recorded in his Bakerian Lecture, 1857, and printed in the 

 ' Philosophical Transactions ' for that year ; and a careful perusal 

 of his observations, and if possible a repetition of his experiments, 

 will well repay the student for his labour. He found that gold-leaf 



* In order to ensure accuracy as to the cause of the colour in these scales, I 

 enlisted the aid of my friend Dr. Frankland, to whom I sent portions of the wings 

 and scales corresponding with those -which I had submitted to microscopical inves- 

 tigation. He bleached, or tried to bleach them with Peroxide of Hydrogen, and 

 with Chlorine water, and the result was generally, as I had anticipated. The 

 brown scales bleaclied easily and comjdetely ; the blue ones only turned pale 

 green. After describing to me the diffeient reactions, Dr. Frankland said : — 

 " Judging from these experiments as well as from the appearance of the wings, I 

 should say that the blue scales owe their coloru" in every case to interference, whilst 

 all the rest are tinted with pigments," 



