RECENT ADVANCES IN PHOTOGRAPHY. 397 



cate plant that is dried and bottled. With the true spirit of an origi- 

 nal investigator, who sees that there is more to be learned than has 

 already been discovered, and with a modesty that well becomes one 

 who has accomplished so much, Dr. Lawes (now Sir John Bennet 

 Lawes) writes me : " We are getting greatly in arrears of what may 

 be called published work, and both Dr. Gilbert and myself are much 

 more interested in searching after the unknown than in making public 

 what little we do know. I think, however, it is not right to keep back 

 so much valuable matter, and I shall try and publish next year the 

 whole series of our ash analysis, without comment of any sort, merely 

 giving the history of the experiments in regard to manures, etc., so 

 that the reader may be able to trace tbe remarkable changes which 

 take place from time to time. You will see that we have got to a 

 point in our experiments in which the mere growth of the crop is one 

 item, and a very small one, in the scope of our inquiry ; the relations 

 of the crop to the manure and to the soil and atmosphere bring us 

 face to face with problems of great difficulty, which require several 

 life-times to elucidate." 



--- 



KECENT ADVANCES IN PHOTOGKAPHY* 



Br Captain ABNEY, E. E., F. E. S. 



TAKING the case of a daguerreotype plate which has been ex- 

 posed, and which we are about to develop by the action of mer- 

 cury, I should like you to understand exactly what takes place in the 

 plate when it is exposed and developed. On the surface of the plate 

 we have a mixture of silver iodide and bromide ; but, for simplicity's 

 sake, I will suppose that it is simply silver iodide. When light acts 

 on such a compound, the result is the liberation of iodine and the for- 

 mation of a new salt, which we call silver subiodide, AgI a = Ag 3 I + 1. 

 The iodine is taken up by the silver plate at the back of the sensitive 

 film. To develop the picture, mercury-vapor is caused to condense on 

 the subiodide, and leave the iodide intact. In the Talbotype process, 

 the picture, which has been taken on a paper that has been washed 

 with nitrate of silver, iodide of potassium, and nitrate of silver again, 

 is developed by washing with gallic acid and silver nitrate. The 

 picture begins to appear on washing after a very short exposure to the 

 light, and becomes gradually more visible as the washing goes on. A 

 paper process is a most fascinating process, because you can dabble 

 about, and do exactly what you like ; it is not like the gelatine plates 

 of the present day, which you have to leave to come out mechanically. 

 With paper, if you want to bring out a little better detail in one place, 



* Abstract of four Cantor Lectures delivered before the Society of Arts. 



