June 8, 1882] 



NA TURE 



H3 



acting along with the diffused light from all parts of the sky, 

 they did more harm than good. They behaved, in fact, like the 

 rays from the lamp in the experiment of Section 6. The slightly 

 inferior luminosity of the parts to some little distance on both 

 sides of that on which the shadow fell, shows that the loss of 

 the diffuse light corresponding to the portion of the sky cut off 

 by the pole was quite sensible when that portion lay very near 

 the sun. 



All this falls in very well w ith what we know of the nature of 

 the direct sunlight and the light from the sky. In passing 

 through the atmosphere, the direct rays of the sun get ob- 

 structed by very minute particles of dust, globules of water 

 forming a haze too tenuous to be noticed, &c. The veil is 

 virtually coarser for blue than for red light, so that in the unim- 

 peded light the proportion of the rays of low to those of high 

 refrangibility goes on continually increasing, the effect by the 

 time the rays reach the earth increasing as the sun gets lower, 

 and has accordingly a greater stretch of air to get through. Of 

 the light falling upon the obstructing particles, a portion might 

 be absorbed in the case of particles of very opaque substances, 

 but usually there would be little loss this way, and the greater 

 part would be diffused by reflection and diffraction. This dif- 

 fused light, in which there is a predominance of the rays of 

 higher refrangibility, would naturally be strongest in directions 

 not very far from that of the direct light ; and the loss accord- 

 ingly of a portion of it where it is strongest, in consequence of 

 interception by the pole in front of the tablet, accounts for the 

 fact that the borders of the place of the shadow were seen to be 

 a little less luminous that the parts at a distance. 



8. The observations on phosphorescence just described have 

 now prepared the way for the explanation I have to suggest of 

 the photographic phenomenon. 



It is known, that with certain preparations, if a plate be ex- 

 posed for a very short time to diffuse daylight, and be then 

 exposed to a pure spectrum in a dark room, on subsequently 

 developing the image it is found, that while the more refrangible 

 rays have acted positively, that is, in the manner of light in 

 general, a certain portion of the less refrangible have acted in 

 an opposite way, having undone the action of the diffuse daylight 

 to which the plate was exposed in the first instance. 



It appears then that in photography, as in phosphorescence, 

 there may in certain cases be an antagonistic action between the 

 more and less refrangible rays, so that it stands to reason that 

 the withdrawal of the latter might promote the effect of the 

 former. 



Now the objective of a photographic camera is ordinarily 

 chemically corrected ; that is to say, the minimnm focal length 

 is made to lie, not in the brightest part of the spectrum, as in a 

 telescope, but in the part which has strongest chemical action. 

 What this is, depends more or less on the particular substance 

 acted on ; but taking the preparations most usually employed, it 

 may be said to lie about the indigo or violet. Such an objective 

 would be much under-corrected for the red, which accordingly 

 would be much out of focus, and the ultra-red still more so. 



When such a camera is directed to a uniform bright object, 

 such as a portion of overcast sky, the proportion of the rays of 

 different refrangibilities to one another is just the same as if all 

 the colours were in focus together ; but it is otherwise near the 

 edge of a dark object on a light ground. As regards the rays in 

 focus, there is a sharp transition from light to dark ; but as 

 regards rays out of focus, the transition from light to dark, 

 though rapid, is continuous. It is, of course, more nearly 

 abrupt the more nearly the rays are in focus. Just at the out- 

 line of the object there would be half illumination as regards the 

 rays out of focus. On receding from the outline on the bright 

 side, the illumination would go on increasing, until on getting 

 to a distance equal to the radius of the circle of diffusion (from 

 being out of focus) of the particular colour under consideration, 

 the full intensity would be reached. Suppose, now, that on the 

 sensitive plate the rays of low refrangibility tend to oppose the 

 action of those of high refrangibility, or say act negatively, then 

 just outside the outline the active rays, being sharply in focus, 

 are in full force, but the negative rays have not yet acquired 

 their full intensity. At an equal distance from the outline on 

 the dark side, the positive rays are absent, and the negative rays 

 have nothing to oppose, and therefore simply do nothing. 



9. I am well aware that this explanation has need of being 

 confronted with experiment. But not being myself used to 

 photographic manipulation, I was unwilling to spend time in 

 attempting to do what could so much better be done by others. 



I will, therefore, merely indicate briefly what the theory would 

 lead us to expect. 



We might expect, therefore, that the formation of the fringe 

 of extra brightness would depend : — 



(1) Very materially upon the chemical preparation employed. 

 Those which most strongly exhibit the negative effect on exposure 

 to a spectrum after a brief exposure to diffuse light might be 

 expected to show it most strongly. 



(2) Upon the character of the light. If the light of the bright 

 ground be somewhat yellowish, indicating a deficiency in the 

 more refrangible rays, the antagonistic effect would seem likely 

 to be more strongly developed, and, therefore, the phenomenon 

 might be expected to be more pronounced. 



(3) To a certain extent on the correction of the objective of 

 the camera. An objective which was strictly chemically cor- 

 rected might be expected to show the effect better thaa one in 

 which the chemical and optical foci were made to coincide, and 

 much better than one which was corrected for the visual rays. 



It is needless to say that on any theory the light must not be 

 too bright, or the exposure too long ; for we cannot have the 

 exhibition (in the positive) of a brighter border to a ground 

 which is white already. 



P.S. — Before presenting the above paper to the Royal Society 

 I submitted it to Capt. Abney, as one of the highest authorities 

 in scientific photography, asking whether he knew of anything 

 to disprove the suggested explanation. He replied that he 

 thought the explanation a possible one, encouraged me to present 

 the paper, and kindly expressed the intention of submitting the 

 question to the test of experiment. 



Linnean Society, May 24. — Anniversary Meeting. — Sir 

 John Lubbock, Bart., F.R.S., president, in the chair. — Mr. 

 H. T. Stainton, on behalf of the Audit Committee, read the 

 statement of receipts and payments for the year, and the 

 Trea-urer, Mr. Frank Crisp, followed with a detailed ex- 

 planation of the various items, showing that the Society was 

 in a very sound financial condition ; besides investments 

 of about 4000/., the balance at bankers' being 649/. 2s. $J. 

 Afterwards the secretary, Mr. B. D. Jackson, read his 

 annual report. Since the last anniversary, fifteen Fellows of 

 the Society, 2 Foreign Members, and I Associate, had died, 

 and 7 Fellows had withdrawn ; while 40 new Fellows had 

 been elected. Between purchase, exchange, and donations, 383 

 vols, and 348 separate parts had been added to the Library. — 

 The President then delivered his anniversary address, comment- 

 ing generally on the events of the past year with especial refer? 

 ence to their bearing upon the Society ; he also made allusions 

 to the removal of the Botanical Department of the British 

 Museum to South Kensington, and to the additions of Miss 

 North's oil paintings, &c, to Kew Gardens ; this was followed 

 by reports on the various botanical and zoological publications 

 published during the last twelvemonth. The obituary notices 

 of deceased Fellows was read by the Secretary, the Society 

 having to deplore the loss of Charles Darwin, Professor 

 Rolleston, Sir C. Wyville Thomson, and their late treasurer, 

 Mr. Frederick Currey, who had been in office above twenty 

 years. The scrutineers having examined the ballot, then 

 reported that Mr. H. W. Bates. T. S. Cobbold, Prof. P. M. 

 Duncan, E. M. Holmes, and Sir J. D. Hooker had been elected 

 into the Council, in the room of Prof. Allman, Rev. J. M. 

 Crombie, W. S. Dallas, A. Grote, and Prof. Lankester, who 

 retired ; and for officers, Sir J. Lubbock as president, Frank 

 Crisp as treasurer, and B. D. Jackson and G. J. Romanes. 



Manchester 



Literary and Philosophical Society, March 13.— Alfred 

 Brothers, F.R.A.S., in the chair.— Mr. Marcus M. Hartog, 

 F.L.S., made a communication upon water-fleas. — On Cypraa 

 guttata (Gmel.), by J. Cosmo Melvill, F.L.S. — Lepidoptera of 

 the Shetland Islands, by Hastings C. Dent, C.E.— Notes on 

 the Giant Dragon's-blood tree at Orotava, by Mr. John Plant, 

 F.G.S.— Mr. R. D. Darbishire, B.A., F.G.S., exhibited a fine 

 series of Ceylonese land and freshwater shells, procured through 

 the instrumentality of Mr. M. M. Hartog, F.L.S. 



April 17. — Annual Meeting. — Mr. Boyd remarked upon 

 the discovery of the egg-cases of Pidiculis capitis in the 

 crevices in an African chief's head stool in the possession of a 

 friend of his. — Mr. Plant stated that he had endeavoured to 

 obtain larger specimens of the Dreissena noted at the last meet- 

 ing, but without success. — Dr. Alcock concluded his notes on 



