OF ARTS AND SCIENCES. 159 



IKVESTIGATIONS ON LiGlIT AND HeAT, MADE AND PUBLISHED WHOLLY OR IN PART WITH 

 ApPllOrBIATION FROM TUB RuMFORD fUND. 



VIII. 



A METHOD OF MEASURING THE ABSOLUTE SENSI- 

 TIVENESS OF PHOTOGRAPHIC DRY PLATES. 



By William IL Pickering. 



Communicated May 14fch, 188-1. 



WiTiiix the last few years the subject of dry pLite photography has 

 increased very rapidly, not only in general popularity, but also in im- 

 portance in regard to its applications to other de[)artments of science. 

 Numerous plate manufacturers have sprung up in this country as well 

 as abroad, and each naturally claims all the good qualities for his own 

 plates. It therefore seemed desirable that some tests should be made 

 which would determine definitely the validity of these claims, and that 

 they sliould be made in such a manner that other persons using instru- 

 ments similarly constructed would be able to obtain the same results. 



Perhaps the most important tests needed are in regard to the sensi- 

 tiveness of the plates. Most plate makers use the wet plates as their 

 standard, giving the sensitiveness of tlie dry plates at from two to sixty 

 times greater ; but as wet plates vary quite as much as dry ones, de- 

 pending on the collodion, condition of the bath, etc., this system is very 

 unsatisfactory. Another method employed largely in England, de- 

 pends on the use of the Warnerke sensitometer. In this instrument 

 the light from a tablet coated with luminous paint just after being 

 exposed to a magnesium light, is permitted to sliiiie through a colored 

 transparent film of graduated density upon tlie plate to be tested. 

 Each degiee on the film has a number, and, after a given exposure, the 

 hist number photographed on the plate represents the sensitiveness on 

 an empirical scale. There are two or three objections to this instru- 

 ment. In the first place, the light-giving power of the luminous tablet 

 is liable to variations, and, if left in a warm, moist place, it rapidly de- 

 teriorates. Again, it has been shown by Captain Abney that plates 

 sensitized by iodides, bromides, and chlorides, wldch may be equally 

 sensitive to white light, are not equally affected by the light emitted 

 by the paint ; tlie bromides being much the most rapidly darkened, 

 the chlorides next, and the iodides least of all. The instrument is 



