110 Lord Rayleigh on the Optical Character 



If We take fju — 1*5, the minimum retardation is represented 

 by -746, the maximum retardation at perpendicularity being- 

 taken as unity. It may be remarked that the minimum 

 retardation maj r practically be secured without pushing very 

 far the obliquity outside. If we suppose the maximum 

 retardation to give a coloration corresponding to the Fraun- 

 hofer line (a, = 6563), the minimum will correspond to 

 A, = 4896, pretty close to the line F. According to the 

 interference theory, then, the range of coloration should be 

 from the full red of C to the blue-green of F, and this is 

 just about what is observed. The agreement must be 

 admitted to be a strong argument in favour of the theory. 

 So far as I have seen, so great a range cannot be found in 

 the surface colour of any dye, even with the aid of polarized 

 light. 



I have already mentioned that the opaque backing behind 

 the seat of coloration can be attacked, and for the most part 

 removed, with nitric acid, so as to allow the transmission 

 colour to be observed. But a much superior effect has been 

 obtained by Dr. Eltringham, using eau de javelle (hypo- 

 chlorite) *. After removal of the backing, the wing-case was 

 mounted with balsam in a slide, which Dr. Eltringham has 

 kindly left in my possession. Close observation of this 

 specimen has yielded results which I think interesting and 

 telling. Seen by transmitted light with the aid of a 

 Coddington lens, the slide shows a pale green over the 

 larger part of the area, which by perpendicular reflexion 

 is a full red. The green is fairly uniform except where it 

 appears perforated with small circular spots, which look 

 reddish, but perhaps only by contrast. Especially to be 

 noted is the fact that there is no colour seen by transmission 

 at all comparable in saturation with those exhibited by 

 reflexion. For observation of the reflected light it is 

 advantageous, though not necessary, to renew the opaque 

 backing, which was done by coating the under surface of the 

 glass with gelatine darkened with ink. A good deal depends 

 upon the source of light. In the first detailed examination, 

 the source (a gas-mantle) happened to be highly localized, 

 and I was puzzled to reconcile the highly spotty character 

 of the reflexion, varying from red to green or green-blue 

 according to the incidence, with the uniformity of the 

 Transmission tint. Similar appearances could of course be 

 observed in direct sunlight. But when the slide was held 



* Dr. Eltringham's label runs : — 



Mimela leei. Elytron after prolonged eau-de-javelle. Only surface- 

 rilm left. Transmits complementary colours to those it reflects, and 

 reflects same colours from both sides" (191 7). 



