The time relations of the photo‐electric changes in the eyeball of the frog

THE photo-electrical changes in the excised eyeball of the frog have been investigated by a large nunmber of physiologists, the most recent work on the subject being that of Waller'. The most interesting and fundamental problem in connexion with the electrical changes produced by the action of light is undoubtedly that of their source, but a question of primary importance and one which must throw light on this problem is that of the time relations of the photo-electric changes. At present this question of the tinme relations appears to be an extremely open one; thus the delay between illumination and eyeball effect is according to Waller one of seconds, according to Dewar and McKendrick one of a few hundredths of a second2, and according to Fuchs only from -a4to TY? of a second3. These discrepancies are due to the impossibility of regarding the movement of a galvanometer needle as a record of the tinme relations of the changes which have produced such movement, and Waller himself recognised this in the published account of his galvanometer observations. As regards the determination mnade by Fuichs, a mlethod was employed which, althougb valuable for nerve and muscle, appears to be singularly inappropriate for the investigation of eyeball effects, and involves two sources of fallacy so grave that his results are deprived of all value as regards this particular question of the time relations of the eyeball changes. The method used by Fuchs was a sparking rheotome; each revolution of the rheotome disc caused a spark in front of the eyeball or retina, and also connected the tissue with a galvanometer for a definite small period of time; the rate of rotation of the rheotome was from 4 to 2 per second, and the rotation was continued from 12 to 15 times with the