A RETINAL MECHANISM OF EFFICIENT VISION 351 



2. Rods. Until an intensity is used at- which printed matter 

 can be read, the rods of Ameiurus remain in the typical short- 

 ened condition of darkness; at about this grade, however, they 

 apparently begin to elongate slightly. Any considerable degree 

 of lengthening must first appear only in stronger light. 



DISCUSSION AND CONCLUSION 



These results, as compared with the findings of Garten and 

 Weiss (compare p. 347), indicate that there exists a rather lower 

 degree of photic sensitivity in the visual cells and retinal pig- 

 ment than they maintain. Nevertheless, it must be apparent 

 to all, not only that in the subjective choice of arbitrary grades 

 of light a variable personal factor enters, but also that the de- 

 scription of stages thus chosen cannot be expressed in accurate 

 terms. It is possible, of course, for any one person to standardize 

 these grades fairly well for his own experiments; on the other 

 hand, the determination of a critical intensity, such as that in 

 which 'colors can be distinguished,' depends on one's individual 

 acuity in color discrimination, on the brightness of the test 

 colors, and on the decision as to whether these colors are to be 

 just distinguishable with intense scrutiny or to be identified 

 with ease. In any event, it appears that my first grade of 

 illumination (that in which objects were discernible) was un- 

 doubtedly of lower intensity than the weakest employed by Gar- 

 ten and Weiss • (compare p. 347) ; it lay far below the point 

 where colors cease to be recognizable, this latter constituting their 

 lowest grade. 



Moreover, it is not impossible that the intensity of light to 

 which their animals were actually subjected was higher than 

 Garten and Weiss supposed. Their animals, contained in a 

 'spacious basin,' received light (electric) reflected downward 

 from the cover. After the experimenter had accustomed his 

 eye to total darkness for five minutes, the condition of illumina- 

 tion was judged by looking downward into the dish at colors 

 placed directly over the surface of the water. As to whether 

 this is a method which tends toward underestimating the light 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 30, NO. 4 

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