May 26, 1899.] 



SCIENCE. 



735 



iologie der Sinnesorgane, XV., pp. 247-279, 

 1S97), the maximum brightness for peri- 

 pheral as for direct vision in bright light 

 being in the yellow ,ajid not, as in the other 

 two classes, in the green. 



According to the theory of Von Kries 

 the visual mechanism used in bright light 

 diifers entirely from that used in faint light. 

 The former distinguishes colors as colors, 

 and finds the greatest brightness in the yel- 

 low of the spectrum, but requires a certain 

 intensity of illumination before it can act 

 at all. The other is insensible to color, 

 seeing the spectrum, as stated above, as a 

 strip of varying brightness, with its max- 

 imum in the green. As one who is born 

 totally color-blind sees the spectrum in the 

 same vfnj, von Kries argues that in this 

 case the ' brightness-apparatus ' is absent 

 or ineffective, and that vision is due en- 

 tirely to the ' twilight-apparatus,' which 

 in the normal eye becomes important only 

 in faint light. 



On the other hand, he goes on to say, the 

 periphery of the normal eye acts " not (as 

 the totally color-blind eye) by means of an 

 absence of the ' brightness-apparatus,' and 

 an exclusive use, therefore, of the ' dark- 

 ness-apparatus,' but through a limitation or 

 change in the functions of the ' brightness- 

 apparatus.' In the language of the ana- 

 tomical hypothesis, we must assume that 

 even in the periphery of the ' brightness- 

 adapted ' eye the cones play the most im- 

 portant part, and that the color-blindness 

 arises from a functional modification of the 

 apparatus depending mainly on these ele- 

 ments, the 'brightness-apparatus.' This 

 view is supported by the fact that the per- 

 iphery values show approximately the same 

 relations in the distribution of brightness 

 as the color-perceiving portions of the eye, 

 with the maximum near the sodium-line." 



As this question is of considerable im- 

 portance in the theory of color-vision, it 

 seemed worth while to re-examine it with 



the flicker photometer, an instrument which 

 appears excellently adapted to such a pur- 

 pose. Its value in the study of ordinary 

 color-blindness was pointed out by the writer 

 in a paper read at the Detroit meeting of 

 this Association, and Professor Rood, work- 

 ing with a flicker instrument of an entirely 

 different type, has come to the same con- 

 clusion. The flicker photometer is also 

 peculiarly adapted to the study of per- 

 ipheral vision, since, as is well known, the 

 peripheral regions of the retina are es- 

 pecially sensitive to appearances of motion 

 or of changing brightness. 



The instrument used in these experi- 

 ments was of the revolving-disk type al- 

 ready described by the writer {Physical 

 Review, Vol. III., ISTo. 16, Jan.-Feb., 

 1896). To this instrument the arc of a 

 circle was attached, the center of which 

 was as nearly as possible the place occu- 

 pied by the eye in front of the observing 

 tiabe. This was marked in three points, 

 at 30, 50 and 70 degrees from the line of 

 direct vision. "When the eye was directed 

 on one of these marks observations with 

 the sight tube could be made at the corre- 

 sponding obliquity. All observations were 

 made in a horizontal plane on the nasal 

 side of the retina. 



The conditions were those of ordinary 

 photometric observation ; the room was 

 dark and the eye screened from any light 

 except that under observation. Thus the 

 eye was , without doubt, partially ' adapted 

 for darkness,' though the lights under ob- 

 servation were too bright to allow this 

 adaptation to go very far. The sources of 

 light were kerosene lamps, provided with 

 Methven slits and burning a special high- 

 grade oil. They were found to burn with 

 great uniformity, but were checked by fre- 

 quent direct observations. The right-hand 

 lamp was used as a standard, was kept al- 

 ways in one place, and used to illuminate the 

 revolving disk. The left-hand lamp illu- 



