J. W. Draper—Phosphorograph of a Solar Spectrum. 178 
Ill. Or roe PrRopaGaTIon oF PHOSPHORESCENCE FROM 
PartTicLE To PARTICLE, 
manner that it does. 
The test plate referred to in the next paragraph was thus 
words were written on 1t. Some photographic 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 
tothe sky. It was then carried into a dark room, and the test 
plate laid upon it. On the test plate another non-shining phos- 
phorographic 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 phosphorescence in- the second 
tablet, through glass more than a millimeter thick. 
_ This Jateral 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 insolation and that 
of observation. 
t 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 circum- 
stances, sharpness is impossible, because the phosphorograph 
itself is not sharp. 
‘or this reason, also, the bright rectangle in a phosphoro- 
graph of the solar spectrum, arising from the coalescence of the | 
infra-red lines a, , 7, 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 somewhat rounded at its corners. 
If we could obtain a thermograph of the solar spectrum, it 
would correspond very closely to the phosphorograpb. The 
particles heated would radiate their heat to adjacent ones, 
