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594                                      ON  THE  OPTICAL  CHARACTER                                     [438
brightness of the reflected light could hardly be explained without supposing more than the two or three alternations which might suffice were air in question. Mr Onslow thinks that there may then be a difficulty in finding room for the alternating structure and the protective covering.
An important question is whether the change of colour with angle is such as can plausibly be attributed to a periodic structure. As Walter points out, a good deal depends upon whether, or not, there is a limitation upon the obliquity of the rays within the thin plate, or plates. In the ordinary arrangement for Newton's rings there is no limitation, the direction in the air-film being parallel to that of the rays before incidence upon the first plate. The optical retardation may then vary from its maximum at perpendicular incidence to zero at 90 obliquity. According to this, it should always be possible to push the colour out of the spectrum at the blue end by sufficiently increasing the obliquity, but it must be remembered that unless .special provision is made the colour effects would be overlaid by the white  light reflected at these angles from the first glass surface encountered.
From what we have seen in the case of the beetle colours where we must suppose that the refractive index does not differ greatly from that (1'6) of the chitin, there is a limit to the obliquity within the thin films even when externally the incidence is grazing. If 6 be the angle in the thin film and ^ the refractive index, the retardation is proportional to cos 0, and in the limiting case
cos 8 =
If we take /u.= l-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 may practically be secured without pushing very far the obliquity outside. If we suppose the maximum retardation to give a coloration corresponding to the Fraunhofer line C (\ = 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 (hypochlorite)*.
* Dr Eltringham's label runs :
Mimela led.    Elytron after prolonged eau-de-javelle.   Only surface-film left.    Transmits complementary colours to those it reflects, and reflects same colours from both sides (1917).ds;  and since the range of relative index is then much restricted, the