BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 88 



than to strengthen the retinal image at low intensities, even at the 

 expense of considerably impaired definition. The human eye seems, 

 however, to have become specialized for considerable acuity in a mod- 

 erate light rather than for such extreme sensitiveness as is found in 

 many nocturnal animals whose pupillary apertures vary over a much 

 wider range than in man. 



The curves of Figure 1 show simple retinal sensitiveness, and in 

 reckoning from them one must at low illuminations take account of 

 the gain from increased aperture. At ordinary working values of the 

 illumination the gain is small, but at 1 or 2 meter-candles it is very 

 material and plays a most important part in practical vision. For 

 example, by curve a, Figure 1, an illumination of 0.5 meter-candle would 

 imply a value of Fechner's fraction of about 0.2, which would in turn 

 imply very much impaired shade-perception. In point of fact, one 

 can see quite tolerably by a candle at the equivalent distance of 1.4 

 meters. 



For if the pupil has adjusted itself to this situation the virtual 

 illumination is that corresponding to about 2 meter-candles, the equiv- 

 alent area of the pupil having increased to at least four times its ordi- 

 nary value, which is that to which the curves of Figure 1 pertain. 

 The result is a value of 0.1 or less for Fechner's fraction, which is 

 quite another matter. 



Were it not for this assistance, it would be quite impossible to get 

 accurate photometric readings at the low intensities common upon the 

 photometer screen. Similarly it would be exceeding difficult to get 

 about at night, even by moonlight. In this latitude moonlight near 

 full moon may fall to about 0.2 meter-candle, which would give Fech- 

 ner's fraction at nearly .5, barring aid from the iris. With this aid 

 increasing the aperture perhaps 6 times, one can see to get about very 

 easily and can even read very large print. The same conditions have 

 an important bearing on vision in presence of a strong radiant. For 

 example, suppose that in a general illumination of 1 meter-candle one can 



make out objects having a contrast -y = .15. Then let a light giving 



20 meter-candles come fairly into the field of vision without materially 

 illuminating these objects. The pupil will close to about one third its 

 former area, giving a virtual illumination of about 0.3 meter-candles 

 and a shade-perception of about .30, in which, of course, the objects 

 disappear. Hence one cannot see well across a bright light, and even 

 objects illuminated by it lose in visibility unless the change in illumi- 

 nation from them is greater than the concomitant change in aperture 

 ratio. 



