of a Solar Spectrum. 159 



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 be- 

 fore they can be discerned. 



III. Of the Propagation of 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 neighbourhood to shine ; and 

 each of these in its turn may excite others ; and so the lumi- 

 nosity may spread. In a former memoir I examined this in 

 the case of chlorophane, and concluded that in that substance 

 such a communication does not take place : but now, using 

 more sensitive preparations, as follows, I have established in 

 a satisfactory manner that it does. 



The test plate referred to in the next parapraph was thus 

 made. A piece of glass was smoked on one side in a flame, 

 until it became quite opaque. When cool a few letters or 

 words were written on it. Some photographic varnish was 

 poured on it and drained. This, drying quickly, gave a black 

 surface which could be handled without injury. 



A phosphor ographic tablet was made to shine by exposure 

 to the sky. It was then carried into a dark room, and the 

 test plate laid upon it. On the test plate another non-shining 

 phosphorographic tablet was laid, and kept in that position a 

 few minutes ; then, on lifting this from the test plate, the 

 letters were plainly visible, especially if it were laid on a piece 

 of hot metal. So the light radiating from the first tablet 

 throuo-h the letters of the test could produce phosphorescence 

 in the second tablet, through glass more than a millimetre 

 thick. 



This lateral illumination is therefore sufficient to destroy 

 the impression that is left by the fixed lines, unless indeed 

 their breadth be sufficiently exaggerated and as short an 

 interval as possible permitted between the moment of insola- 

 tion and that of observation. 



It has been remarked that a photograph taken from a phos- 

 phorograph is never sharp. It looks as if it were taken out 

 of focus, and this even though it may be a copy by contact. 

 The light has spread from particle to particle. Under such 

 circumstances, sharpness is impossible, because the phospho- 

 rograph itself is not sharp. 



