158 Dr. J. W. Draper on the Phosphorograph 



senting three regions : — 1. A middle one, extending from the 

 boundary of the blue and green to a little beyond the violet ; 

 in this region the argentic iodide is blackened. 2. Below 

 this, and extending from the boundary of the blue and green 

 to the inferior theoretical limit of the prismatic spectrum, is a 

 region strongly marked, in which the action of the daylight 

 has been altogether arrested or removed, the daylight and the 

 sunlight having apparently counterbalanced and checked each 

 other. 3. A similar protected region occurs beyond the 

 violet ; this, however, is very much shorter than the prece- 

 ding. The sketch annexed to Herschel's paper represents these 

 facts as well as they can be by an uncoloured drawing. 



II. Description of the Phosphor ographic Spectrum. 



In a phosphorograph on luminous paint the same general 

 effects appear. If the impression of the spectrum be taken 

 in the absence of extraneous light, there is a shining region 

 corresponding to the blackened region of the photograph. 

 But if, previously or simultaneously, extraneous light be per- 

 mitted to be present, new effects appear. The shining region 

 of the phosphorograph has annexed to it, in the direction of 

 the less-refrangible spaces and extending toward the theoreti- 

 cal limit of the spectrum, a region of blackness in striking- 

 contrast to the surrounding luminous surface. This blackness, 

 however, is broken at a distance below the red by a luminous 

 rectangle of considerable width. This occupies the space, and 

 indeed arises from the coalescence, of the bands a, fi, y, dis- 

 covered by me in 1842. It may be separated into its con- 

 stituent bands, which are very discernible when registered 

 on gelatine as presently described. And since this is not so 

 easily done with the upper lines of the spectrum, we may in- 

 fer that these are very much broader than the Fraunhofer lines, 

 a result strengthened by the fact that these dark intervals can 

 be more easily recognized by the thermopile than those lines. 

 The blackness is then resumed. It extends to a short distance ; 

 and there the phosphorographic impression comes to an end. 



This shining rectangle has long been known to students of 

 phosphorescence ; but its interesting origin has not until now 

 been explained. 



But more, just beyond the region of the violet, the same 

 kind of action occurs — a dark space, which, however, is of 

 very much less extent than that beyond the red. 



The photograph and the phosphorograph thus present many 

 points of similarity. But though there are these striking 

 points of resemblance, there are also striking differences. 



