TIUNSACTIONS OK SECTION A. 487 



There is one other recent advance which has been made in scientific photography 

 to which I may be permitted to allude, viz. that from being merely a qualitative 

 recorder of the action of light it can now be used for quantitative measurement. I 

 am not now alluding to ])hotographic actinometers, such as have been brought to 

 such a state of perfection by lloscoe— but what I allude to is the measurement and 

 interpretation of the density of deposit in a negative. Bv making exposures of dif- 

 ferent lengths to a standard light, or to difiereut known "inten.^ities of light on the 

 same plate en which a negative has to be taken, the photographic values of the light 

 acting to produce the densities on the ditferent parts of the developed image cau"be 

 readily found. Indeed, by making only two difterent exposures to the same light, or 

 two exposures to two different intensities of light, and applying the law of density of 

 deposit in regard to them, a curve is readily made from which the intensities of light 

 necessary to give tlie different densities of depo.-it in the imago impressed on the 

 same plate can be read off. The application of such scales of density to astro- 

 nomical photographs, for example, cannot but be of the higliest interest, and will 

 render the records so made many times more valuable than they have hitherto 

 been. I am informed that the United States astronomers have already adopted 

 the use of such scales, which for tbe last three years I have advocated, and it 

 may be expected that we shall have results from such scaled photographs which 

 will give us information which would before have been scarcely hoped for. 



One word as to a problem whicli we may say is as yet only qualitatively and not 

 quantitatively solved. I refer to the interchaiigeability of length of exposure for 

 intensity of^ light. Tut it in this way. Suppose, with a strong liirht, L, a short 

 exposure, E, be given, a chemical change, C, is obtained : will the same change C be 

 obtained if the time is only an wth of the light L, but n times the exposure ?" Now 

 this is a very important point, more particularly when the body acted upon is fairly 

 stable, as, for instance, some of the water-colour pigments, which are kniown to fade 

 in sunshine, but might not be supposed to do so in the light of an ordinary room, 

 even with prolonged exposure. Many experiments have been made at South 

 Kensington as regards this, more especially with the salts of silver, and it is found 

 that for any ordinary light, intensity and exposure are interchangeable, but that 

 when the intensity of light is very feeble, say the y^^^ of ordinary daylight, the 

 exposure has to be rather more prolonged than it should be, supposing the exact 

 interchangeability always lield good ; but it has never been found that a light was 

 so feeble that no action could take place. Of course it must be borne in mind that the 

 stability of the substance aefed uj)on may have some ellect ; but the same results 

 were obtained with matter which is vastlv more stable than the ordinary silver 

 salts. It may be said in truth that almost'all matter which is not elemental is, in 

 time and to some degree, acted upon by light. 



I should like to have said somethiug regarding the action of light on the iron 

 and chromium salts, and so introduced the sul'ject of platinotvpe and carbon 

 printing, the former of wliich is creating a revolution in tlie production of artistic 

 prints. I have, however, refrained from so doing, as I felt that tlie Presidi'iit of 

 Section A should not be mistaken as the President of Section B. Photogravure 

 and the kindred processes were also inviting subjects on which to dwell, more 

 especially as at least one of tiiem is based on the use of the same material as that 

 on whicli the first camera picture was taken by Niepce. Again, a dread of trench- 

 ing on the domains of art restrains me. 



Indeed, it would have been almost impossible, and certainly impolitic, in the 

 time which an address should occupy, to have entered into the many branches of 

 science and art which i)hotography covers. I have tried to confine myself to some 

 few advances that have been made in its theory and practice. 



The discovery of the action of light on silver salts is one of the marvels of this 

 century, and it is dilHcult to overrate the bearing it has had on the progress of 

 science, more especially i)hysical science. The discovery of telegraphy took i)lace 

 in the present reign, and two years later photography was pracfieally'introduced; 

 and no two discoveries have had a more marked infiuence on mankind. Telegraphy, 

 however, has had an advantage over photography in the scientific progress that it 

 has made, in that electrical cmrents are subject to exact measurement, and that 



