February i6, 1922] 



NATURE 



221 



nautical Research Committee to obtain facilities for 



full-scale experiment on R31, R32, and R33 had 



jfailed. He urged as a partial atonement for the 



jcrifice of the lives of brave men that the remnants 



the airship fleet should be devoted to correcting, by 



ill-scale experiments, the knowledge given by model 



tperiments. 



The situation with regard to aeroplanes was also 



lid to be unsatisfactory. Prof. L. Bairstow pointed 



the prevalence of accidents in aviation, the risks of 



ring being so great that life insurance companies are 



prepared to cover them in a standard policy. This 



ibnormal rate was attributed in part to a number of 



jfects not needing more knowledge for their remedy. 



"le failure of rubber joints in petrol pipes and the 



iakage of ignition cables were given as instances of 



isily remediable defects. The danger of such imper- 



;tions comes from the necessity for landing at once, 



len the engine ceases to turn, in country which is 



Ften unsuitable. So soon as an aeroplane can take 



ie air it leaves the designer for the user, and its 



»inor defects have not then developed. New design 



ight be encouraged by placing responsibility for such 



itters on the designer, and not on an Air Ministry 



iff. 



Other causes of failure in flight were said to need 



lore knowledge before they could be removed. All 



roplanes are tricky at low speeds, and it does not 



accord wholly with fact to attribute an accident _ to 

 "bad piloting causing the aeroplane to lose flying 

 speed near the ground." Important research work is 

 here called for, the conduct of which may be hindered 

 by an unsuitable organisation as much as by lack of 

 funds. It is reported that the Secretary of State for 

 Air has the matter of the reorganisation of the Air 

 Ministry under review, and has afforded the council 

 of the Royal Aeronautical Society an opportunity for 

 expressing the scientific and technical view of essential 

 requirements for the carrying on of research. It is 

 encouraging to note that Lord Gorell told the Air 

 Conference that " success in the air, whether Service 

 or civil, must depend primarily upon constant scientific 

 research," and that the report of the Geddes Economy 

 Committee, whilst asking for a reduction on the vote 

 for experiment and research, accepts that view. 



If research and care in design can reduce the acci- 

 dents now occurring to one-third of their present 

 amount — a very moderate estimate of early possibili- 

 ties — the saving of money on flying risks, replacement 

 of aircraft, etc., will very greatly exceed the cost 

 incurred. The position is unusual in the fact that the 

 effects of research on the final product are so clearly 

 seen ; such a state is largely due to the infancy of the 

 science, but an important additional element arises 

 from the unparalleled degree of freedom of the motion 

 of aircraft as compared with that of other vehicles. 



The Grain of the Photographic Plate. 



'X* HE unit of the photographic plate is the single 

 -*■ grain of silver salt as it exists in the sensitive 

 film. It is natural, therefore, that after many years 

 and much labour had been devoted to the properties 

 of sensitive films as films, attention should be turned 

 to the unit. A great deal of work has already been 

 done in this direction, but many problems, some of 

 which seem to be of an elementary character, remain 

 to be solved. 



On Tuesday, February 14, Prof. The Svedberg, of 

 Upsala, communicated to the Royal Photographic 

 Society two papers containing important results that 

 he has obtained. It is customary in such investiga- 

 tions to dilute the emulsion and so produce a film 

 that contains only a single layer of grains. The 

 characters of the grains are registered by photomicro- 

 graphy, using apochromatic objectives of the maxi- 

 mum practical aperture. As the photography of the 

 grains before treatment must not affect their sensitive- 

 ness. Prof. Svedberg used a very deep red light and 

 Ilford special rapid panchromatic plates. After ex- 

 posure and development the plate may be photo- 

 graphed again, then either the metallic silver pro- 

 duced or the unaffected grains may be dissolved 

 away, as desired, to facilitate the examination of the 

 remainder. 



Evidence in favour of the view that the halide 

 grain is either wholly reducible (developable) or not 

 reducible at all is accumulating, and this Prof. Sved- 

 berg finds definitely to be the case. By dissolving 

 away the silver grains nothing whatever was left of 

 them, except to the extent of about i per cent, of 

 the thousand or so grains observed, which showed 

 traces of incomplete reduction (develooment). 



This independency of the grains is further proved 

 by the unchanged appearance of the undeveloped 

 grains and their unchanged sizes as measured. This 

 holfls even when the film is partly solarised by a 

 st^'ong light, when the grains are separated by only 

 lu, and whether ferrous oxalate or metol-hydroquinone 

 developer is employed. The author intends to try 

 NO. 2729, VOL. 109] 



other conditions to see if, as appears to be the case, 

 these results are general, and that feeding of the 

 reduced silver grains at the expense of the undevelop- 

 able grains does not, in fact, take place. 



In Prof. Svedberg 's second communication he sug- 

 gests that the larger and the smaller grains in one 

 and the same emulsion are equally sensitive and "are 

 built up of the same kind of light-sensitive material — 

 just as if they were fragments of different sizes from 

 one homogeneous silver bromide crystal." He 

 assumes that by exposure (light action) "developable 

 centres " are produced, and shows experimentally that 

 the distribution of these "centres" takes place ac- 

 cording to the laws of chance, so that there is no 

 need to assume a superior sensitiveness of those 

 grains that are made developable. The author isto 

 be congratulated on using the term "centre," which 

 expresses all that is known and is non-committal, 

 rather than "nucleus." Nuclei have been shown to 

 serve, but the crystallisation of sugar on strings is 

 not evidence of the presence of strings wherever sugar 

 crystallises. This by the way. 



The present writer in 191 1 (Journ. Roy. Phot. Soc, 

 p. 159) showed that by stopping development at a 

 very early stage it was possible to get particles of 

 silver too small to be visible microscopically. They 

 were shown to be present by the colour imparted to 

 the film, and were further demonstrated and measured 

 by adding mercury to them in known and progressive 

 proportions and measuring the enlarged particles. 

 Prof. Svedberg by stopping development at a little 

 later stage gets particles that are just definitely visible 

 microscopically, and shows photographically the rela- 

 tion of these to the original grams of silver haloid. 

 He thus demonstrates that "centres of development " 

 are produced by exposure. A single developable grain 

 may contain one or more (so far up to four) of these 

 "centres." He treats also of other matters, such as 

 the effect of Rontgen rays when used instead of 

 ordinary light. 



C.J. 



