July, 1912. 



KNOWLEDGE 



277 



bination with the gelatine and indirectly give rise to the 

 foruialion of bodies which have an equally deleterious action 

 upon the deveiopablo ini.ijje. It is now thirty vears ago since 

 Sir William .\bney in one of a course of Cantor lectures 



Figure 306. 



(1882) delivered before the Society of .'\rts, dealt with this 

 subject of reversal, showing a film which had received an 

 exposure of one minute behind a negative to direct sunlight. 

 and yet the image obtained was quite free from any effects of 

 reversal. This was explained as being due to the film having 

 been treated with potassium nitrite which took up the bromine 

 as fast as it was formed, and so prevented it from attacl<ing 

 the silver bromide that had been altered by light. .\nd to use 

 this scientist's own words : if you want to get rid of reversal 

 you must give the plate something which will very rapidly 

 absorb bromine, and which if possible is not organic. What 

 was foreshadowed as being possible so many years ago, has 

 now been practically realized by the discovery that certain 

 salts or derivatives of hydrazine possess the required properties, 

 and plates coated with emulsion containing these bodies have 

 been placed upon the market by the Paget Prize Plate Com- 

 pany Limited, under the name of "" Hydra " plates. By the 

 courtesy of the Paget Company the writer has been able to 

 try these plates, with the most gratifying results — subjects from 

 which, owing to their great difi"erence in brilliancy, it had been 

 impossible to obtain satisfactory results previously, photo- 

 graphed most perfectly, there being not the slightest trace of 

 reversal, and the invisible backing with which the plates are 

 coated preventing halation, even with very prolonged exposures. 

 The e.xamples which form the illustrations to this article will, I 

 think, speak for themselves, Ifigure 306 being a photograph of 

 an incandescent electric light, taken on an ordinary rapid 

 gelatine plate, while Figure 307 is the same lamp photographed 



under the same conditions on a Paget " Hydra " plate. In 

 Figure 306 the image of the filament is entirely reversed, while 

 in Figure 307 there is no sign of reversal, beyond which the 

 increased brilliancy (luminosity) given with the " Hydra" plate 

 is quite evident as well. The speeds of the plates employed 

 were 250 H. and D. and the exposures given were forty 

 minutes with stop F 22 although we found that complete 

 reversal was obtained on the ordinary plate in ten nnnutos. 

 The developer employed in both cases was a normal pyro- 

 soda one, used in everyday work, so that no departure was 

 made from ordinary procedure. When the degree of over 

 exposure is very great, " some forty-times," then a special 

 developer has to be used. This, however, is supplied in a 

 convenient form by the makers of the plates. 



EXPOSURE TABLE FOR JULY.— The calculations 

 are made with the actinograph for plates of speed 200 H. and 

 D., the subject a near one, and lens aperture F.16. 



Remarks. — If the subject be a general open landscape, take half 

 the e.vposures given here. 



