Time necessary to excite the Human Retina. 347 



however, it is necessary to use some method of only illumi- 

 nating the mirror at intervals for less than a complete 

 revolution, thus ensuring that only one flash reached the eye 

 tor each light interval. This end was brought about by 

 interposing a revolving disk, with a sector of suitable size 

 cut out, between the lens and the radiant. The dimensions 

 of the sector removed from the disk were arranged so that 

 when the disk was running at about 120 r.p.m. the length 

 of flash should be about ~ sec, which would be less than 

 one revolution of the centrifuge. The frequency of the 

 flashes seen by the observer in this case was about two per 

 second. To drive the disk, it was mounted directly on the 

 shaft of a variable speed repulsion motor. As a matter of 

 fact, in practice the disk was driven at a slightly greater 

 speed than 120 r.p.m. This increased the frequency of the 

 flashes slightly, but diminished the chance of any light from 

 two successive revolutions of the mirror reaching the eye. 



The optical arrangements were also improved. New 

 lenses of various focal lengths were obtained from Adam 

 Hilger & Co. Finally a lens of about 4 metres focal length 

 was found to answer most satisfactorily. If a longer focal 

 length than this is used, the question of obtaining sufficient 

 illumination in the beam becomes rather troublesome. 

 Different forms of radiant were also tried. The plan of 

 using a slit as radiant was abandoned as sufficient light could 

 not be obtained from it. After trying various other light 

 sources, a small galvanometer electric lamp supplied by the 

 Cambridge & Paul Scientific Instrument Co. was found to 

 work most satisfactorily. This lamp had a single loop 

 filament, and by turning this loop parallel to the direction 

 of the light-beam, a very fair approximation to a line source 

 of light could be obtained. 



The method of observing the flashes was as follows. One 

 observer took up his position at the eye-slit, which was fitted 

 with a small tube so that the direction of vision might be 

 preserved constant in the dark. The other observer then 

 started the centrifuge and ran it up to the required speed. 

 In the meanwhile the revolving sector shutter was kept 

 permanently open, so that what the first observer perceived 

 was a steady light in the mirror. When he had satisfied 

 himself that this light was visible, i.e. that the direction of 

 the light-beam was correct, he gave a signal, and the sector 

 shutter was started. This shutter was not, however, run 

 continuously, but it was stopped in the open position occa- 

 sionally, so that the observer might satisfy himself that the 

 light-beam was reaching his eye correctly. This precaution 



