BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 87 



marked shading off at the edges, something as if a slit were being 

 closed. The appearances indicate that beside the undoubted aberra- 

 tions which come into play, there is considerable spreading of light in 

 the retina at high intensities, reinforced very likely by reflection from 

 the choroid, producing an effect quite analogous to the halation observed 

 in a photographic plate. 



The dimensions of the irradiation effect thus observed are inferior to 

 the dimensions required by Charpentier, but it is quite probable that 

 with a dark-adapted eye and feeble illumination, lessened contrast with 

 the chief image would render the outlying portions more conspicuous. 



The increased visibility of rather large areas is a still more puzzling 

 matter, for which no satisfactory explanation has been produced. Inas- 

 much as all dealings like these with threshold sensibility have by this 

 condition eliminated the cones of the retina from action, and depend 

 upon rod vision entirely, it may be, since the rods are relatively more 

 numerous away from the fovea, that mere size of image insures its 

 falling on retinal areas relatively rich in active visual elements. 



Aside from questions of intensity in artificial illumination is the 

 matter of steadiness. It is of course well known that violent transi- 

 tions of light and darkness, whether by moving the person or the eye, 

 or by changing the intensity of the light itself, are distressing and 

 injurious. The retina has a certain amount of visual inertia, which 

 furnishes protection against very rapid changes, else one could not use 

 lights successfully with alternating current. Flicker, from a practical 

 standpoint, is troublesome about in direct proportion to its magnitude 

 and in inverse proportion to its frequency. A change of intensity, how- 

 ever, covering some seconds, giving the iris plenty of time for readjust- 

 ment, is hardly noticeable, while one of the same numerical magnitude, 

 say 20 per cent each side of the mean, occurring once or a few times 

 per second, is most painful. Ordinary incandescent lamps run on alter- 

 nating current vary from 5 to 15 per cent on each side of the mean, 

 according to the thermal inertia of the filament, and the frequency. 

 V\ ith lamps of ordinary voltage and candle power the flickering is per- 

 ceptible at between 20 and 30 cycles per second, the new high-efficiency 

 lamps being worse than the older ones. Practically all lighting is 

 done at above 30 ~, and troublesome flickering comes only from the 

 irregular fluctuations of bad service. It must not be forgotten that one 

 can impress serious fluctuations of light on the retina by compelling the 

 eye to confront great variations of illumination when it moves. No 

 artificial light should be arranged so that it forces the eye to make 

 sudden transitions from blackness to brilliancy. Figure 3 is given here 

 as a horrible example of what should never be permitted. I am sorry 



