224 PROCEEDINGS OF THE AMERICAN ACADEMY 



II. Desceiption of the Phosphorographic Spectrum. 



In a phosphorograpli 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 black- 

 ened region of the photograph. But if, jDreviously or simultaneously, 

 extraneous light be permitted to be present, new effects appear. The 

 shining region of the phosphorograph has annexed to it, in the direc- 

 tion 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 is, however, 

 broken at a distance below the red by a luminous rectangle of consid- 

 erable width. This occupies the space, and indeed arises from the 

 coalescence of the bauds a, (3, y, discovered by me in 1842. It may 

 be separated into its constituent 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 

 infer 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 phos- 

 phorographic impression comes to an end. 



This shining rectangle has long been known to students of phos- 

 phorescence, but its interesting origin has not until now been ex- 

 plained. 



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 resem- 

 blance, there are also striking differences. 



In a spectrum four or five centimetres long, though the photograph 

 may be crossed by hundreds of Fraunhofer lines, not one is to be seen 

 in the phosphorograph, except those just referred to. The spectrum 

 must be dispersed much more before they can be discerned, 



III. Of the Propagation op Phosphorescence from 



Particle to Particle. 



The explanation of this disappearance of the Fraunhofer lines is 

 obvious. A phosphorescing particle may emit light enough to cause 

 others in its neighborhood to shine, and each of these in its turn may 



