Photo-Electric llieory of Vision. 355 



was then measured in the usual way. The advantage of 

 using lead disks to place the active material on is that it 

 makes the manipulation of the latter easier, and also, when 

 one preparation has been tested, the disk can be quickly 

 removed from the testing vessel and a second one sub- 

 stituted. Lead is a convenient metal to use, as it is easily 

 cut to the required shape, and is also very inert piioto- 

 electrically. 



The experiments carried out on the black pigment with 

 this apparat is completely confirmed the mil results previously 

 obtained with it. Various methods of preparing the pigment 

 were tried, it was tested moist and after being dried in a 

 desiccator, immediately after dissection and after the lapse 

 oE some time, but in no c;ise was any photo-electric effect 

 obtained. A test was also made on a complete retina, which 

 was spread entire on one of the lead disks, but it also showed 

 no photo-electric powers. All these experiments were con- 

 ducted at a pressure of about 5 mm. of mercury. There 

 can, it seems, be very little doubt that the black pigment of 

 the eye is not photo-electric when exposed to visual light. 



As regards the rhodopsin in the retina, it was thought best 

 to test it in situ in the retina. This was done by testing a 

 freshly prepared retina obtained from a frog's eye. The 

 frog, before the experiment, had been kept in the dark, 

 so that there was a certain amount of rhodopsin present 

 in the retina. Professor Pringle kindly did the requisite 

 dissections which were carried out in feeble red light, 

 the retina not being exposed to ordinary illumination at all 

 tiil it was tested. As it was considered that perhaps the 

 vacuum in the testing vessel might tend to render the retina 

 inactive owing to its drying action, these trials were all 

 carried out at air pressure. 



The result of these experiments showed that while the 

 frog's retina certainly possesses no permanent photo-electric 

 effect, there is usually a very small effect for a few minutes 

 after the retina is first placed in the testing vessel. In all 

 cases such an effect was found, but its size was very small 

 and irregular, varying from about 6 x 10 " 6 E.8.U. to 

 0-5xl()- 6 E.S.U. 



In view of the small size of the currents obtained, it is 

 perhaps of interest to consider what would be the smallest 

 number of electrons per second which the human eye could 

 be expected to detect. If we assume that a candle is visible 

 at night at a distance of 3000 metres, and that the effective 

 aperture of the eye in such a case would be about 1 sq. cm., 

 we find that the minimum visible energy-How is equal to 



