OF ARTS AND SCIFNCES. 225 



excite others, atul so the himinosity may spread. In a former rcemoir 

 I examined tliis in the case of chlorophane, and concluded tliat in that 

 substance such a communication does not take place. But now, using 

 more sensitive preparations, as follows, I have established in a satis- 

 factory manner that it does. 



The test plate referred to in the next paragraph 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 j^hotographic varnish was poured on it and drained. This, 

 drying quickly, gave a black surface which could be handled without 

 injury. 



A phosphorographic 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 through the letters of the test could produce phospho- 

 rescence in the second tablet, through glass more than a millimetre 

 thick. 



This lateral illumination is therefore sufficient to destroy the im- 

 pression 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 insolation and that of observation. 



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

 graph 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 phosphorograph itself is not sharp. 



For this reason, also, the bright rectangle in a phosphorograph of the 

 solar spectrum, arising from the coalescence of the infra-red lines 

 a, ^, y, is never sharp on its edges. It seems as if it were fading 

 away on either side. It is also broader than would correspond to the 

 actual position and width of those lines, and, particularly, it is some- 

 what rounded at its corners. 



If we could obtain a thermograph of the solar spectrum, it would 

 correspond very closely to the phosphorograph. The particles heated 

 would radiate their heat to adjacent ones. Nothing like sharpness of 

 definition could be obtained, except m very brief exposures Delore 

 the effect had had time to spread. 



