72 Mr. J. S. Dow on the Physiological 



the rod-flicker can be regarded as distinct in other respects 

 from the cone-flicker. 



In the first place there seems to be, not only a difference 

 in the critical speed o£ disappearance, but also in character. 

 Several physiologists have pointed out that two varieties o£ 

 flicker seem to exist. At low speeds the flicker-sensation is 

 of a violent, coarse nature, but, as the speed is increased, this 

 becomes modified and eventually gives way to a fine trembling 

 variety, and this it is which we make use of, as a rule, in the 

 flicker photometer. 



If the speed is increased still further this, too, eventually 

 disappears as all the impressions succeed each other too 

 rapidly for flicker of any kind to be seen. From some 

 observations the author is inclined to associate the violent 

 variety of flicker with the rods and the finer variety with the 

 cones. It seems that the violent variety is the only kind 

 which can be properly seen when the illumination is very low, 

 so that presumably only the rods would be in action. Under 

 these conditions we observe the usual peculiarities connected 

 with rod- vision. For instance, the violent flicker is best seen 

 by averted gaze, so as to bring into action tbe peripheral 

 region of the retina where the rods are most numerous. 

 (This circumstance seems to have led Dr. Porter to make an 

 ingenious suggestion. He supposes that in prehistoric ages 

 the peripheral region of the retina so developed as to be more 

 sensitive to flicker than the central region (although other 

 senses are there most acute), in order to enable men to gain 

 immediate intimation of any moving object in the neigh- 

 bourhood such as might constitute a possible source of 

 danger.) 



At high illuminations, however, when the speed is adjusted 

 to a value such that the fine variety of flicker is produced, the 

 exact opposite may be the case, i. e. we may find it possible to 

 observe the flicker by the aid of the central region of the 

 retina when it cannot be seen peripherally. Very interesting 

 charts of the sensitiveness of different regions of the retina 

 can be produced at different speeds. The author, however, 

 found that, at high illuminations, the flicker, with increasing- 

 speed, tends to disappear last near the centre of the retina. 

 This therefore favours the suggestion that this fine flicker, 

 such as occurs at the higher speeds, is best seen by the aid of 

 the cone organs. 



r L Note. — In this connexion the following experiment (which 

 was shown on the occasion of the reading of this paper) is of 

 interest. Two disks of equal size are prepared, and can be 

 driven from the same motor, side by side, with the same 



