274 M. H. Vogel on the Sensibility to Light 



millim. The solar rays were thrown on to it by means of a 

 Foucault's heliostat, for the use of which I am indebted to the 

 great kindness of my friend Dr. Zenker. The length of the 

 spectrum from D to G was 35 millim s. 



When desirous of comparing my experiments, I chose the 

 time from 11 till 2, and then operated only when there was a 

 cloudless sky, which it is true is but seldom the case at this 

 period of the year. The time of exposure lasted generally ten 

 minutes. The plates were developed with protosulphate of iron. 

 Even in my earliest experiments I found that the sensibility of 

 the bromide of silver extends considerably further than is stated 

 by Dr. Schultz-Sellack, who obtained therewith an action only 

 from the ultra-violet as far as nearly to the line F in the blue. 

 In my experiments the sensibility extended in all cases beyond 

 the line F, more or less far beyond according to the transparency 

 of the atmosphere. Upon this latter point I purpose entering 

 more at length in a separate communication. 



I made a trial of the bromide of silver in two forms :— -first, as 

 a so-called wet plate, that is to say, when moist from an adhe- 

 ring solution of nitrate of silver derived from the silver-bath 

 wherein the plates were sensitized ; secondly, as a dry plate, ob- 

 tained by washing off the solution of silver and then drying the 

 plate. (For further details see { Reports of the German Chemical 

 Society/ 1873, p. 89.) The behaviour of the two classes of 

 plates was different. 



The result was that dry bromide of silver exhibits a more ex- 

 tended sensibility for colours than does the bromide which lies 

 beneath a silver solution ; when an acid development was em- 

 ployed, the latter manifested sensibility up to the middle point 

 between D and E — that is to say, nearly up to the yellow ; the 

 former, however, exhibited sensibility 2 millim s. beyond the line 



D, or up into the orange. 



The action of the two plates was moreover very different in a 

 qualitative sense. With moist bromide of silver an extremely 

 energetic action is seen between G and F (in the indigo and 

 blue) ; near F, however, it declines suddenly, and nothing but a 

 faint indication could be traced as far as the other side of 



E. Dry plates, on the other hand, exhibited a far less decided 

 action in the blue than the wet plates did; this action,. however, 

 declined only gradually, and it extended, as has been remarked 

 above, as far as beyond D. 



Hence dry bromide of silver is more sensitive for the rays of 

 lesser refrangibility in the visible solar spectrum ; moist bromide 

 of silver is most sensitive for the blue rays of greater refrangi- 

 bility. 



For ordinary photographic plates a solution of silver acts as a 



