The Electrical Response of the Eye to Stimulation by Light 385 



not hold good, and with larger retinal areas the amounts of energy required 

 increase proportionately with the square root of the area. 



We may expect that analogous rules exist for the photo-electric reaction 

 of the isolated frog's eye. In our experiments the flux of light on the retina 

 was often diminished by the use of diaphragms from 1 to 10"^. When 

 once it has been diminished to 10"^ or 10'* the spot of light on the retina 

 has in all probability become so small that the quantity of light required 

 for a photo-electric reaction has become independent of the area of the spot. 



Four groups of diaphragms were prepared. Those of the first group could 

 be placed close to the collimator lens at D^, while at Dg the diaphragms 

 from the three other groups served respectively for the three parts of the 

 spectrum used by us. It need scarcely be mentioned that all diaphragms were 

 applied centrically round the optical axis. The diameters of the diaphragms, of 

 which the size was exactly controlled by aid of the microscope, were so chosen 

 that the successive members of a series each weakened the light by ten times. 



The following are the dimensions of the diaphragms used: — 



Table I. 

 Circular Diaphragm Di. 



Table II. 

 Diaphragm D.^. 



