﻿the Reciprocity Law in Photography. 909 



of accuracy in the timing. The plates were fully developed 

 in hydroquinone and caustic soda developer, and "cut" 

 by a few seconds' immersion in potassium ferricyanide 

 to remove the slight chemical fo«; and to give a more 

 easily measurable end to the line. After this treatment, 

 the Paget, and in one or two instances the W ratten, plates 

 could be measured directly, but in other cases the plates had 

 to be printed onto Paget':?, and one, or sometimes two, further 

 transparencies made before it was possible to measure them. 

 The plates from which the wedge constant was determined 

 were treated in the same manner. All measurements were 

 made on a Hilger travelling microscope, and are accurate to 

 *1 mm., and in some cases to '01 mm. 



As is well known, irregularities occur near the edges of a 

 plate. For this reason, only two long and two short lines 

 were made on each plate, and thus only the central portions 

 were used. 



It will be seen that the results for the same type of plate 

 differ appreciably, but this is to be accounted for by variations 

 in the thickness of the film on the plate, and the errors 

 arising in this manner are eliminated by taking the mean of 

 a considerable number of determinations on different plates. 



Paget Plates. 



Wratten Plates. 



Imperial Plates. 



Exposure 

 (seconds). 



V- 



Exposure. 



V- 



Exposure. 



p. 



32 & 15976 



0-830 



10 & 1020 



0-874 



20 & 2000 



0-843 







10 & 1000 



0-840 





0-843 



32 & 15976 



0-833 













0-900 



10 & 1000 



0-867 



10 & 1000 



0-868 







10 & 1020 



0-861 





0-880 



60 & 6000 



0-867 













0879 



10 & 1000 



0-857 

 0-798 



10 & 1000 



0-861 

 0-897 



60 & 6000 



856 













0-873 



10 & 1000 



0867 

 0-834 



10 & 1000 



0-854 

 0-861 



45 & 4500 



0-792 













817 



10 & 1000 



0-831 

 0-840 



10 & 1000 



0-765 

 0-830 



45 & 4560 



0962 













0-902 



5 & 550 



0-878 



10 & 1000 



0-847 

 0-837 



45 & 4500 



0-861 



10 & 1000 



0-834 









0-871 





0-815 



10 & 1000 



0-803 





0-865 









0-865 



45 k 4500 



0-847 







10 & 1000 



0-829 

 0-846 



30 & 2000 



0-869 











30 & 2000 



0885 

 0-868 











