386 nutting: chromatic aberration of human eye 



there was placed immediately in front of the eye a plate of optical 

 glass, P, reflecting the image of a suitable object at the desired 

 distance. 



Three eyes were tested at accommodations of 25 cm., 100 cm., 

 and °° ; the remainder at °° only. The object serving to fix the 

 accommodation was so chosen that the slit image appeared against 

 a dark background, such as a distant tree trunk or a black printed 

 letter, so that a barely perceptible amount of white light was 

 mixed with that of the colored slit image. In making determi- 

 nations at wave lengths 406 and 436 a mercury arc was used as 

 source, at other wave lengths a Nernst lamp. A shift of the lens 

 1 cm, corresponded to 0.01 mm. shift in focal point at the retina, 

 the relation between the two being roughly linear. The uncer- 



-< 



Fig. 1 



tainty on a group of five settings was not over 2 mm.; results 

 could be reproduced at different times to about 5 mm. on the 

 scale. 



All the subjects chosen had good normal vision. Two, C. H. B. 

 and C. E. S., were women. Four, L. A. J., P. G. N., C. E. S., 

 and M. B. H., were experienced in photometric observations; the 

 others, practically without experience in such work. L. E. J. 

 has marked natural ability and has had long experience in color 

 work. 



The results obtained are given in the following table and in 

 figure 2. Data are given as relative focal differences, 5v/v. 

 From these, approximate distances in milhmeters of focal points 

 from the retina may be obtained by multiplying by 15. 



All the eyes tested show more or less correction. For compari- 

 son, the axial error of an eye of pure water is given at the top of the 

 figure. In the most luminous part of the spectrum, from 520 to 



