July 13, 1876] 



NATURE 



239 



bottom — and I think a very creditable depth too— they 

 found that, while the surface of the water might be full of 

 these calcareous organisms, the bottom was not. There 

 they found that red clay so pathetically alluded to by my 

 friend on the right [Commander Stewart, who replied for 

 the Navy] as the material to which when glory called him 

 he might be reduced. This red clay is a great puzzle — a 

 great mystery — how it comes there, what it arises from, 

 whether it is, as the director has suggested, the ash of 

 foraminiferae ; whether it is decomposed pumice-stone 

 vomited out by volcanoes, and scattered over the surface, 

 or whether, lastly, it has something to do with that 

 meteoric dust which is being continually rained upon us 

 from the spaces of the universe — which of these causes 

 may be at the bottom of the phenomenon it is very hard 

 to say ; it is one of those points on which we shall have 

 information by-and-by. I will not detain you further 

 with speaking of the matters of interest which have come 

 out of this cruise of the Challenger; I will only in con- 

 clusion remind you that work of this kind could by no 

 possibility be done without the zealous aid of an intelli- 

 gent executive. That is the first condition, but our thanks 

 have already been rendered to the executive officers of 

 the Challenger. In the second place, it could only have 

 been done by the aid of such a scientific staff, composed 

 of picked men as was sent out in the Challenger, such 

 men as Buchanan, Murray, and Moseley, and Wild, and 

 Suhm ; and I can hardly mention the name of the last 

 gentleman without, in passing, lamenting that he alone of 

 all the staff" who left our shores, — he who certainly was 

 the last person we should have imagined we should not 

 see again — that a man of his accomplishments and pro- 

 mise and geniality and lovability should be the only one 

 not to be welcomed back by the friends who loved him, 

 and by the country which would have been glad to adopt 

 him. But, again, a work such as has been done by the 

 Challenger could only have been effectively carried out 

 under the direction, not only of a man who mtellectually 

 knew what he was about, but whose moral qualities were 

 such as to get the people with whom he was associated 

 to work with him." 



Prof. Huxley concluded by referring to the harmony 

 which throughout prevailed among the staff" of the Chal- 

 lenger. 



" When men are shut up together in a limited society, 

 whether it be a cathedral town or a ship, they begin to 

 hate one another unless the bishop is a very wise person. 

 In this case I do not doubt that the bishop was a very 

 wise person, and I do not believe that the whole course 

 of the Challenger afforded occasion for any such trian- 

 gular duels as one hears of in the novels of Captain 

 Marryat." 



Sir C. Wyville Thomson made a suitable reply to the 

 toast, giving a brief account of the various operations of 

 the Challenger, and referring to the great amount of work 

 yet to be done ere all the results could be given to the 

 world. 



PHOTOGRAPHIC PROCESSES 1 



TT is not my intention to enter into the history of any of the 

 ■*■ processes to which I propose to call your attention to-night, 

 as I somewhat dread to enter upon such controversial ground. 

 Probably the demonstration of the production of photographic 

 prints by various methods will be of greater interest than any 

 history. 



Astronomy was the religion of the world's infancy, and it can 

 hardly be a matter of surprise that untutored yet inquiring minds, 

 unaided by any distinct revelation, should have attributed to the 

 glorious orb, the centre of our solar system, the possession of 

 divine attributes, and as they gazed upon the wondrous effects 

 of his magical painting, that they should have offered to him 

 their adoration and worship, and carefully noted any phenomena 



' Lecture by Capt. Abney, R.E., F.R.S., at the Loan Collection, South 

 Kensington. 



due to him. Thus probably the first photographic action 

 noticed would be at a very early period of human existence, 

 when the exposure of the epidermis to his rays caused what is 

 known to us as tan, whilst the parts of the body covered would 

 remain of their pristine whiteness. A photographic action 

 which would be remarked at a later date would be the fading of 

 colours in the sunlight. Ribbons, silks, curtains, and similar 

 fabrics of a coloured nature undergo a change in tint when ex- 

 posed to it. 



I have here a specimen of a pink trimming used by the fair 

 sex, and the lady who presented me with it informed me that it 

 was "a most abominable take in," as the colour "goes" after 

 two days' wear. Her ideas on the subject and my own somewhat 

 differed, for to me it presented a capital opportunity of using 

 the material as a means for obtaining a photographic print in a 

 moderate time. I have here two results of the exposure of this 

 stuff to the sunlight. One was exposed beneath a negative of an 

 anatomical subject, and we have the image represented as white 

 upon a pink ground. The other subject is a map. An ordinary 

 map was superposed over a square piece of the stuff, and placed 

 in sunlight whilst in contact. We have in this case the lines of 

 the map represented as pink on a white ground, from which the 

 colour had faded. 



The general opinion is, I believe, that the colour is given off 

 somewhat similarly to the scent from a rose. Were this entirely the 

 case, the light would not act as it does, but beneath the negative 

 or map, the colour would bleach uniformly. The bleaching 

 seems to be a really chemical change in the dye due to the 

 impact of light. There are many other bodies besides dyes 

 which change in light, and some of them are of the most un- 

 likely nature. I had intended to show you to-night the change 

 that takes place in glass by expoiure to light for long periods. 

 My friend, Mr. Dallmeyer, has in his possession specimens of 

 brown and flint glass, which have markedly changed colour in 

 those halves of the prisms purposely exposed to solar influences. 

 In some cases there is a " yellowing " of the body, and in others 

 a decided " purpling." 



It is, however, only those bodies which change rapidly in the 

 light that are utilised in photography. The most common 

 amongst these are various compounds of silver, for they are 

 peculiarly sensitive to the action of light. Nearly every silver 

 compound is more or less changed by it, and when I say changed I 

 mean altered in chemical composition. When we reflect what light 

 is we can better understand its action. Light, as experiment, 

 confirmed by mathematical investigation, tells us, is caused by a 

 series of waves issuing from the luminous source, not, indeed, 

 trembling in our tangible atmosphere, but in a subtler and in- 

 finitely less dense medium, which pervades all space, and which 

 exists even in the interior of the densest solids and liquids. 

 These waves of ether, as this medium is called, batter against 

 and try to insinuate themselves amongst the molecules of any 

 body exposed to their action, a'good many millions of millions 

 of them impinging every second against it. Surely it is not sur- 

 prising to think, small though the lengths of these waves be, 

 that this persistent battering should in some instances be able 

 to drive away from each of the molecules some one of the atoms 

 of which they are composed. 



Take as a type that salt of silver which was, perhaps, the first 

 known to change in the presence of light-silver chloride. For 

 our purpose v/e may represent each of its molecules as made up 

 of two atoms of silver locked up with two atoms of chlorine. 

 Let us consider the action of the light on only one molecule. 

 The waves strike against it energetically and persistently ; the 

 swing that the molecule can take up is not in accord with the 

 swing of the ether. It is shaken and battered till it finally gives 

 up one atom of chlorine ; the vibration of the remaining two 

 atoms of silver and one of chlorine are of a different period, and 

 are not sufficiently in discord to cause a further elimination of an 

 atom. The molecule which contains the two atoms of silver and 

 one of chlorine is called a sub-chloride of silver or argentous 

 chloride, and is of a grey violet colour. If, then, I place silver 

 chloride (held in position by a piece of paper) beneath a body, 

 part of which is opaque and part transparent, and expose it to 

 sunlight, I shall find that where the opaque parts cover it, there 

 the white chloride will remain unchanged, whilst on the portions 

 beneath the transparent parts, the dark silver sub-chloride will 

 have been formed. Of course were the paper, after removal of 

 the body, to be further exposed to light, the image obtained 

 would disappear, as a blackening over the whole surface would 

 ensue. In this state, then, the print is not permanent. Fortu- 

 nately for photography, a ready solvent of silver chloride was 



