410 Eanthoven and Jolly 



used, and also a pair of Nicol's prisms which can be rotated upon each 

 other. The brightnesses used are expressed in stars of known magnitude, 

 as these are seen by a telescope of definite dimensions. We reproduce a 

 table wliich gives for different star magnitudes the reaction times of the 

 observer H B, in thousandths of a second. 



Observer H.B. 



m. m. n». m. m. m. m. m. m. m. m. IP. 



Star-magnitude 3-7 4-5 52 59 60 6-6 74 8-2 8-3 90 9-4 97 

 Reaction time 253 253 271 285 276 288 332 375 351 450 514 605 



From these figures it is seen that for the observer H.B. the reaction time 

 in the case of weak light of the star magnitude 9*7 m. is considerably greater 

 than that of strong light of the star magnitude 3 7 m. The difference 

 amounts to 0*605 -0253 = 0352 sec. 



If w6 assume that the light perception c)n stimulation by strong light 

 takes place instantaneously, the latent period for the perception of the weak 

 light here used amounts to 0352 sec. As however the perception of strong 

 light must also take some time, we must conclude that the latent period for 

 the perception of the weak light here used is greater than 0352 sec. 



Bakhuyzen also mentions the results which have been obtained by 

 some other observers in an analogous wuy. We would draw attention in 

 particular to E.B. II. where, for a star magnitude 50 m., a reaction time of 

 0*279 sec. was found, and for a star magnitude 98 m. a reaction time of 

 0'815 sec., and who therefore shows a latent period for the perception of 

 weak light which is greater than 0-815 — 0*279 = 0536 sec. 



Particular interest attaches to the observations of Pihl,^ who has 

 calculated the latency of light perception from direct observations of stars. 

 For the methods employed by him we refer to his detailed communication, 

 and need only mention here that Pi hi finds, for the latent period of the 

 perception of weak light, amounts which exceed a full second. 



Considering what we have quoted it need not surprise us that we have 

 been able to determine latent periods for the photo-electric reaction of the 

 isolated frog's eye amounting to 2 sec. 



IV. The Energy of the Stimulation in Absolute Measurement. 



It may not be devoid of interest to know the amount of energy of the 

 radiation employed by us in absolute measurement. Our original intention 

 to determine this amount by direct bolometric observations required to be 

 given up for several reasons, but nevertheless we can form a fairly good 

 idea of it if we attempt to calculate it in accordance with the known laws 

 of radiatioft. 



We can identify the crater of an arc light as regards its radiation 



* The stellar cluster 7 Persei micrometrically surveyed, Christiania, 1891. 



