MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 



The distances of the chlorophane and lamp from the 

 paper were, therefore, as 1:5. The illuminating effect is 

 as the squares of those numbers, and therefore 1 : 25. 

 But for extinction it requires that one light should be 

 sixty times as intense as the other ; it follows, therefore, 

 that at those distances the illuminating power of the 

 lamp is fifteen hundred times as intense as the illumi- 

 nating effect of the spar. 



But the quantity of spar used in this experiment ex- 

 posed a surface much greater than that of the flame ; it 

 w r as estimated to be at least twice as great. This, there- 

 fore, would bring us to the conclusion that the intrinsic 

 brilliancy of the chlorophaue is not ^-^ part that of 

 the lamp. 



This experiment was several times repeated. Thus it 

 was found that the lamp extinguished the shadow from 

 the spar when the relative distances were 1 : 4. The 

 lamp at 4 was therefore sixty times as luminous as the 

 spar at 1 ; that is, their illuminating power was as 1 : 960. 

 But it was estimated that the surface of the spar was 3| 

 times that of the flame of the lamp, so this would make 

 the intrinsic brilliancy -^Vs? a resu ^ f the same order 

 as the preceding. 



From this we conclude that the intrinsic brilliancy of 

 phosphori is very small ; a fine specimen of chlorophane, 

 at its maximum of brightness, yielding a light three thou- 

 sand times less intense than the flame of a very small oil- 

 lamp. 



It was stated above that these photometric experi- 

 ments put a new view on the \vhole subject; in fact, 

 they explain all the difficulties of the foregoing inquiries. 

 How could w r e expect to be able to measure the heat of 

 phosphorescence? The radiant heat of the little oil- 

 lamp here employed would require at such distances a 

 very delicate thermometer to measure it. Is it likely, 



