2l8 



SCIENCE. 



[Vol. XIV. No. 347 



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Vol. XIV. 



NEW YORK, September 27, rSSg. 



No. 347. 



CONTENTS: 



The COMMITTEE ON SITES for the world's fair in New York in 

 1892 has recommended that the upper part of Central Park be 

 taken for the purpose, to which shall be added some outlying un- 

 occupied land. The point of special interest just now is that a 

 considerable portion. of the New York community object to any 

 part of the park being used, maintaining that the upper portions 

 are the most attractive of all, and, as is freely acknowledged by 

 all, that these will have to be denuded of trees arid scraped down 

 to a more level surface by the city contractors, so as to ruin their 

 beauty for a generation to come. It is to be said, also, that the 

 lower parts of the park would be turned practically into little more 

 than an entrance to the fair-grounds proper, thus depriving them 

 of the character which draws so many to them on all holidays. 

 The scheme has its advocates, however ; and it is of course true 

 that a part of its support is to be traced to the interests of real- 

 estate owners, as would be the case if any other site were chosen, 

 only here, the interests being the greater, the support is the more 

 earnest. 



MODERN PHOTOGRAPHY.i 

 The occupant of this chair has a difficult task to perform, should 

 he attempt to address himself to all the various subjects with 

 which this section is supposed to deal. I find that it has very often 



1 Address of Capt. W. De W. Abney, president of the section of mathematics and 

 physics, of the British Association for the Advancement of Science, delivered before 

 the association at its meeting recently (from Nature). 



been the custom that some one branch of science should be touched 

 upon by the president ; and I shall, as far as in me lies, follow this 

 procedure. 



This year is the jubilee of the practical introduction of photog- 

 raphy by Daguerre and Fox Talbot, and I have thought I might 

 venture to take up your time with a few remarks on the effect of 

 light on matter. I am not going into the history of photography, 

 nor to record the rivalries that have existed in regard to the vari- 

 ous discoveries that have been made in it. A brand-new history 

 of photography, I dare say, would be interesting, but I am not the 

 person to write one; and I would refer those who desire informa- 

 tion as to facts and dates to histories which already exist. In 

 foreign histories perhaps we English suffer from speaking and 

 writing in a language which is not understanded of the foreign 

 people ; and the credit of several discoveries is sometimes allotted 

 to nationalities who have no claim to them. Be that as it may, I 

 do not propose to correct these errors or to make any reclamations. 

 I leave that to those whose leisure is greater than mine. 



I have often asserted, and I again assert, that there should be no 

 stimulus for the study of science to be compared to photography. 

 Step by step, as it is pursued, there will be formed a desire for a 

 knowledge of all physical science. Physics, chemistry, optics, and 

 mathematics are all required to enable it to be studied as it should 

 be studied ; and it has the great advantage that experimental work 

 is the very foundation of it, and results of some kind are always 

 visible. I perhaps am taking an optimist view of the matter, seeing 

 there are at least twenty-tive thousand living facts against my 

 theory, and perhaps not one per cent of them in its favor. I mean 

 that there are at least twenty-five thousand persons who take 

 photographs, and scarcely one per cent who know or care anything 

 of the " why or wherefore " of the processes, so far as theory is 

 concerned. If we call photography an applied science, it certainly 

 has a larger number who practise it, and probably fewer theorists^ 

 than any other. 



He would be a very hardy man who would claim for Niepce, 

 Daguerre, or Fox Talbot the discovery of photographic action on 

 matter. The knowledge that such an action existed is probably as 

 old as the fair-skinned races of mankind;, who must have recog- 

 nized the fact that light, and particularly sunlight, had a tanning 

 action on the epidermis ; and the women, then as now, no doubt 

 took their precautions against it. As to what change the body 

 acted upon by light underwent, it need scarcely be said that noth- 

 ing was known ; and perhaps the first scientific experiment in this 

 direction was made rather more than a hundred years ago by 

 Scheele, the Swedish chemist, who found, that, when chloride of 

 silver was exposed to light, chlorine was given off. It was not till 

 well in the forties that any special attention was given to the action 

 that light had on a variety of different bodies ; and then Sir John 

 Herschel, Robert Hunt, Becquerel, Draper, and some few others, 

 carried out experiments which may be termed " classical." Look- 

 ing at the papers which Herschel published in the " Philosophical 

 Transactions " and elsewhere, it is not too much to say that they 

 teem with facts which support the grand principle that without 

 the absorption of radiation no chemical action can take place on a 

 body : in other words, we have in them experimental proofs of the 

 law of the conservation of energy. Hunt's worK, " Researches on 

 Light," is still a text-book to which scientific photographers refer, 

 and one is sometimes amazed at the amount of experimental data 

 which is placed at our disposal. The conclusions that Hunt drew 

 from his experiments, however, must be taken with caution in the 

 light of our present knowledge, for they are often Vitiated by the 

 idea which he firmly held, that radiant heat, light, and chemical 

 action, or actinism, were each of them properties, instead of the 

 effects, of radiation. Again : we have to be careful in taking seri- 

 ously the experiments carried out with light of various colors when 

 such colors were produced by absorbing media. It must be re- 

 membered that an appeal to a moderately pure spectrum is the only 

 appeal which can be legitimately made as to the action of the vari- 

 ous components of radiation, and even then the results must be 

 carefully weighed before any definite conclusion can be drawn. 

 No photographic result can be considered as final unless the ex- 

 periments be varied under all the conditions which may possibly 

 arise. Colored media are dangerous as enabling trustworthy con- 



