[ 345 ] 



XXXIII. An Attempt to determine whether a Minimum Time 

 is necessary to excite the Human Retina. By J. H. J. 

 Poole, Sc.D* 



IN a letter to 'Nature/ published on April 7tli, 1921, 

 Sir Oliver Lodge suggested that possibly there might 

 be a certain minimum time necessary for a beam of light 

 to liberate an electron from a photo-electric surface. Thus, 

 assuming that the area of wave-front which can contribute 

 energy to a very small resonator, like the electron, is of the 

 order of V/V, he showed that for ordinary sunlight the 

 surface would have to be illuminated for about ^ sec. 

 before an electron could be liberated. It seems, however, 

 improbable that such a comparatively large interval of time 

 should be necessary to excite an electron, especially as 

 photographs, in which the action of the light on the sensitive 

 film is probably due to the liberation of such electrons, can 

 be taken in ordinary sunlight with exposures shorter than 

 this. 



As a matter of fact, at the time that Sir Oliver Lodge 

 published the above suggestion in ' Nature/ Dr. Joly and I 

 had already undertaken some expei'iments with the view to 

 seeing if any such effect held for the human retina. The 

 general principle of the method is described by Dr. Joly in 

 a letter to ' Nature ' for April 14th, 1921, but perhaps a 

 recapitulation of it is desirable. 



The general idea of the method was to cause a parallel 

 beam of light to revolve very rapidly by reflecting it from 

 the surface of a rotating mirror. The revolving beam was 

 then viewed through a narrow slit placed at a considerable 

 distance from the mirror ; and it is obvious that if the slit 

 and the light-beam can both be kept narrow, then the 

 resulting flash, seen by the eye, may be made very short 

 indeed by running the mirror at a high speed,- and also by 

 making the distance between the mirror and the slit large. 



In order to get as high a speed of rotation of the mirror 

 as possible, it was mounted on the top of the vertical spindle 

 of a Legendre Centrifuge. This centrifuge was capable of 

 speeds up to about 8000 r.p.m., as was determined with a 

 revolution counter and stop-watch. The mirror itself was 

 a small speculum metal one. of about 4 cms. diameter, 

 manufactured and ground optically plane by Sir Howard 

 Grubb. Duriug the course of the experiments this mirror 



* Communicated by Prof. J. Joly, F.R.S. 



