34& Minimum Time necessary to Excite ike Human Retina. 



is very necessary, owing to the fact that between the very 

 feeble flashes that are visible when the revolving shutter 

 is running, there is a great tendency for the direction of the 

 eye to alter, as in the dark intervals there is nothing to fix 

 its position very exactly. 



It is also very necessary that the dark adaptation of the 

 eye should be nearly perfect. For this purpose the observer 

 should be at least twenty minutes in the dark before 

 attempting to make any trials. The difference in sensitivity 

 between ordinary daylight vision and fully dark-adapted 

 vision is very large. Selig Hecht, in a paper published in 

 the ' General Journal of Physiology ; for May 1920, states 

 that the dark-adapted eye is easily from 5,000 to 10,000 times 

 more sensitive. To attain the maximum sensitivity it is 

 necessary to be at least 45 minutes in the dark, but after 

 25 minutes the increase in sensitivity is very slow, hence 

 probably the period of 20 minutes adopted in these experi- 

 ments was sufficient to ensure that the eye would be nearly 

 fully dark-adapted. 



As regards the results obtained with this apparatus, it 

 was found that a flash of 2 x 10 ~ 7 sec. was still visible. 

 A flash of duration 8 x 10 ~ 8 sec. was, however, found to be 

 invisible. This flash was, however, found to be visible if 

 viewed directly without a slit, in which case the time of 

 flash would be about 24 x 10~ 7 sec, thus confirming the 

 previous result. The energy in each of these flashes could 

 be approximately calculated from the cnndle-power of the 

 radiant and the various dimensions of the apparatus, and it 

 was found that, neglecting any loss by reflexion, etc., the 

 energy in the visible flash was about 4 x 10 ~ 7 erg, while 

 that in the invisible flash was about 8 x 10 " 8 erg. As to 

 whether the invisibility of the shorter flash is due to its 

 smaller duration alone or simply to the decrease in the 

 energy available, is uncertain. It seems, however, that the 

 latter hypothesis is more probable as the intensity of the 

 longer flash was excessively feeble. The problem of getting 

 more light into a fla^h of this nature is not very easy, as the 

 amount of light which can be obtained depends only on the 

 brightness of the radiant, and it is not very easy to get a 

 suitable one which would be briohter than an electric light 

 filament. An arc lamp might have given more light, but 

 would not have been so convenient as a slit, and a condensing 

 lens would have been required. Probably the increase in 

 brightness would also not have been very large. 



Iveagh Geological Laboratory, 

 Sept. 1921. 



