June 17, 1922] 



NA TURE 



795 



The Photographic Plate. 



T^HE third Hurter and Driffield memorial lecture 

 *- was delivered in the theatre of the Royal 

 Society of Arts, before the Royal Photographic Society, 

 on May 9, by Prof. The. Svedberg of Upsala, who took 

 for his subject " The Interpretation of Light Sensi- 

 tivity in Photography." After a short general 

 discussion of light sensitiveness from a purely photo- 

 liemical point of view, particularly with regard to 

 instein's law of the photo-chemical equivalent, the 

 cturer distinguished between ^/a^e-sensitiveness, 

 . tm-sensitiveness, and sensitiveness of the silver- 

 ilide material of the grain. The first is the sensitive- 

 ness that concerns the practical photographer, but 

 the third, together with some purely physical circum- 

 stances, determines the quality of the plate, and is 

 that to which the emulsion maker should devote most 

 (jf his attention. It has been recognised only quite 

 recently that there is a sensitiveness of the haloid 

 which is independent of the physical properties of the 

 film and of the size of the grains or particles. 



From a statistical point of view, and with single 

 (inulsions, that is emulsions prepared at once and 

 unmixed with other emulsions, there is a certain 

 relationship between sensitiveness and the size of the 

 particle, the larger grains being more sensitive than 

 ^]ie smaller. But though the probabiUty for a grain 

 ■ become developable is greater in the case of a large 

 lain than in the case of a smaller one, yet taking any 

 two individual grains one cannot tell whether, on 

 exposure, the larger or the smaller will be the first to 

 become developable. This is accounted for by the 

 fact that development sets in at discrete points in the 

 halide grain and progresses from these points until 

 the whole of the grain is converted into metallic silver. 

 One such starting-point is sufficient to render the 

 grain completely developable. 



These centres of action are located and counted 

 by stopping development at a very early stage, and 

 superposing a photograph of them on a photograph 

 of the same particles taken in a deep red light before 



the exposure. A statistical study of the distribution 

 of these centres gives the interesting result that 

 within each size-class of halide grains they are dis- 

 tributed according to the laws of chance. The 

 lecturer adds, " Whether the developable centres are 

 pre-existent in the grains in the form of especially 

 light-sensitive points, or whether their number and 

 position in a grain is entirely determined by the light 

 only — eventually by the haphazard distribution of 

 the light quanta — we are so far not able to tell." 



For various size-classes of grains in the same 

 emulsion, the average number of centres increases 

 with the size of the grain. This leads to the assump- 

 tion that as a rule all the grains of a single (unmixed) 

 emulsion are built up of the same silver halide 

 material of the same light sensitiveness, the larger 

 grains being more sensitive than the smaller ones 

 merely because of the greater probability that a larger 

 grain may contain one, or more than one, developable 

 centre. The average number of centres is found to 

 be proportional to the surface area of the grains. 



By the use of X-rays, which are so little absorbed 

 that the grain is exposed to their action through its 

 whole mass, the available centres are found to be on 

 the surface of the grain, so that the sensitiveness of a 

 grain is determined entirely by its surface layer. By 

 finding the number of centres per unit area of grain 

 surface it is possible to find the sensitiveness of the 

 silver halide material of an emulsion, independently 

 of the size of its grains, and this has been done 

 for three different emulsions, demonstrating clearly 

 the great differences in sensitiveness to light of the 

 haloid bromide material in different emulsions. Such 

 estimations are likely to be of value to the emulsion 

 maker in his endeavour to prepare emulsions of new 

 and desirable properties. 



The lecturer concluded his discourse by pointing out 

 many questions that still remain to be answered in 

 order to perfect our knowledge of the photographic 

 plate and its sensitiveness to light. C, J. 



Agricultural Research at Aberystwyth. 



^PHE new agricultural buildings and the Welsh 

 ^ plant - breeding station of the Agricultural 

 Department, University College of Wales, Aberyst- 

 wyth, were formallv opened on May 20 by the Minister 

 of Agriculture, the Rt. Hon. Sir Arthur Griffith 

 Boscawen, Bart. 



The Welsh plant-breeding station owes its origin 

 to the foresight of Sir Laurence Philipps, Bart., of 

 Llanstephan House, Boughrood, Radnorshire, who 

 generously provided an endowment of 10,000/. for 

 the purpose, and who further assists the station with 

 an annual donation of 1000/. to its funds for a period 

 of ten years. In 1920 the station was recognised by 

 the Ministry of Agriculture as a research institution 

 entitled to grants-in-aid from the Development Fund, 

 and by virtue of a capital grant and annual grants- 

 in-aid, in addition to Sir Laurence Philipps's generous 

 endowment, it has been possible to equip the station 

 in a thorough and up-to-date manner. 



The work in connection with the new agricultural 

 buildings and the plant-breeding station was started 

 in 1919. The buildings are now completed and consist 

 of commodious and well-equipped laboratories for 

 research in agricultural botany and agricultural 

 chemistry, as well as lecture rooms, and a library. 

 The laboratories of the plant-breeding station are also 

 arranged for in the buildings and have been specially 

 equipped in a manner suitable for the researches 

 which are in progress. The buildings occupy the site 



NO. 2746, VOL. 109] 



of the old foundr\'^ near the station — practically all 

 the laboratories and lecture rooms being, in fact, 

 part of the original building — the alterations having 

 been skilfully made by Mr. Bassett. In addition to 

 the self-contained building which is now solely 

 occupied by the Agricultural Department, Nant- 

 cellan Farm, Clarach, has been acquired for the use 

 of the teaching department, and Frongoch Farm for 

 the Welsh plant-breeding station. The former is 

 situated in the Clarach Valley about three miles 

 from Aberystwyth. It comprises about 142 acres 

 of pasture and arable land together with about 28 

 acres of woodland. The main object of the farm is to 

 furnish faciUties for giving demonstrations to students 

 and for carrying on experiments and research. It is 

 considered eminently suitable for the purpose for 

 which it was obtained. The latter is a farm of 92 

 acres and is used entirely for experimental purposes 

 in connection with the Welsh plant-breeding station. 

 In addition, the plant-breeding station has about 5 

 acres of garden ground situated a few minutes' walk 

 from the laboratories. At the gardens the equip- 

 ment consists of a large span greenhouse and an up-to- 

 date pot culture station, together with cages and 

 other essentials. • 



The formal opening marked a great advance in 

 the facilities afforded in Central Wales for both the 

 student and for research in the problems influencing 

 productivity in the Principality. The investigations 



