548 KEPOBT— 1891. 



I should indeed he floriy to he judged by the performance of mj' own students, but I 

 fear that many of the less obvious mistakes made by reasonably trained examination 

 candidates are more directly traceable to their teachers than some of ns as teachers 

 •would like to admit. 



The change in the Tefrangibility of light by reason of tlie motion of its 

 source, thou^rh familiar enough now, was at first regarded as too small to be 

 observed, and one or two attempts directed to detecting the effect of this principle 

 on the spectra of the stars, or sometimes on sunlight reflected by a 45° mirror into 

 the line of the earth's motion (which is not a possible method), wholly failed. I take 

 pleasure in remembering that this etfect was clearly observed for the firet time by the 

 gentleman we this year honour as our President ; and that it is by this v«ry means 

 that the latest sensational discovery in astronomy of the rapidly revolving twin 

 star ^-Aurig.'E, by Prof. Pickering and the staif connected with the Draper 

 Memorial, was made. 



The funds for the investigation that led to this result were provided by Mrs. 

 Draper, as a memorial to her late husband ; and if 3-Aurigfe does not constitute a 

 satisfactory memorial, I am at a loss to conceive the kind of tombstone which the 

 relations of a man of science would prefer. 



The fourth event to which it behoves me to refer is the practical discovery of 

 a physical method for colour photography. When I say practical I do not mean 

 commercial, nor do I know that it will ever become applicable to the ordinary 

 business of the photographer. Whether it does or not, it is a sound achievement 

 by physical means of a result which the chemical means hitherto tried failed, some 

 think necessarily failed, to produce. I say practical, because already it had been 

 suggested as possible theoretically; and a step toward it, indeed very near it, had 

 been actually made. The first suggestion of the method, so far as I know, was 

 made by Lord Rayleigh in the course of a mathematical paper on the reflection of 

 light, and with reference to some results of Becquerel obtained on a totally difterent 

 plan. Ht^ said in a note that if by normal reflection waves of light were converted 

 into stationary waves, the}' could shake out silver in strata half a wave-length 

 apart, and that such strata would give selective reflection and show iridescence. 



The colour of certaiu crystals of chlorate of potash, described in a precise 

 manner by Sir George Stokes,' and also the colours of opal and ancient glass, tad 

 been elaborately and completely explained by Lord Rayleigh on this theory of a 

 periodic structure (the laminated structure in the case of chlorate of potash being 

 caused by twinning^) ; and be subsequently illustrated it with sound and a series of 

 muslin discs one behind the other on a set of lazy-tongs. Each membrane re- 

 flected an inappreciable amount, but successive equidistant membranes reinforced 

 each other's action, and the entire set reflected distinctly one definite note, of wave- 

 length twice the distance between adjacent muslins. So also with any series of 

 equidistant strata each very slightly reflecting. They should give selective reflection, 

 and the spectrum of their reflected beam should show a single line or narrow band, 

 corresponding to a wave-length twice the distance of the strata apart. ^ 



' Proc. Roy. Soc. Feb. 188.1. ' Phil. Mag. Sept. 1888, pp. 256 and 241. 



' The footnote of Lord Rayleigh on page 158, Phil. Mag. 1887, vol. xxiv., is brief 

 and forcible enough to quote in full :— ' A detailed experimental examination of the 

 various cases in which a laminated structure leads to a powerful but highly selected 

 reflection would be of value. The most frequent examples are met with in the organic 

 world It has occurred to me that Becquerel's reproduction of the spectrum in 

 natural colours upon silver plates may perhaps be explicable in this manner. The 

 various parts of the film of subchloride of silver with which the met.al is coated may 

 be conceived to be subjected during exposure to statimiary luminous waves of nearly 

 definite wave-length, the effect of which might be to impress upon the substance a 

 periodic structure occurririg at intervals equal to half the wave-length of light ; just 

 as a sensitive flame exposed to stationary sonorous waves is influenced at the loops, but 

 Tiot at the nodes {Pliii. Mag. March 1879, p. 153). In this way tlie operation of any 

 kind of light would be to produce just such a mollification of the film as would cause 

 it to reflect copiously that particular kind of light. I abstain at present from de- 

 veloping this suge^estion, in the hope of soon finding an opportunity of making my- 

 self experimentally acquainted with the subject.' 



