370 S. P. Langley — Energy and Vision. 



In this table we have first, the wave-lengths corresponding 

 to the observed angles of deviation, these values reaching from 



0""35 in the ultra-violet to 0^*77 near Fraunhofer's A on the 

 extreme border of the visible red. It is to be observed, how- 

 ever, that the great mass of the observations which were taken 



without disturbing the slit reach from 0^*40 in the deep violet 



to 0^-70 in the deep red. The figures corresponding to 0"*35, 



0^-38, 0^-75, 0^-77 are extremely difficult to obtain with pre- 

 cision and are given here as supplementary to the others. 

 There are four observers : — 



S. P. L., whose eye is somewhat long-sighted (making conven- 

 ient the use of convex glasses of \ meter focus) and not sensitive 

 to very feeble light ; eyes otherwise believed to be in normal 

 condition. 



F. W. V., near-sighted, using glasses whose negative focus is 

 14 cm . The eye appears to be much less sensitive to the red than 

 to the violet. The retina of this eye is somewhat deficient in 

 black pigment. 



B. E. L., near-sighted, using glasses whose negative focus is 

 42 em . 



E. M., a boy of fifteen whose sight is perfect as far as known. 



It will be remembered that throughout this table from 0"*40 



to 0^*70 the light enters through a slit whose aperture is con- 

 stant. If under these conditions the logarithm table can be just 

 read when the slider is one meter from the second slit (s 2 ), the 

 light would be represented by unity ; if at two meters, by 4 ; 

 if at three meters, by 9 ; and so on. As, however, we have 

 already explained, the length of the red being limited to but 

 little over three meters, for the higher values we are obliged 

 to introduce the photometer wheel. For instance, the strong- 

 est light observed by F. W. V. was in the prismatic yellow- 

 green corresponding to a wave-length of 0"-55 where 193*8 was 

 noted. Had the rod been really indefinitely prolongable, the 

 slider would have needed to have been removed to the length 

 of nearly 14 meters. To avoid this the photometer wheel was 

 interposed, reducing the light to -fa and the actual distance of 



/193'8 

 the slider from the slit (s 2 ) was, as we may easily see, k/ 



or 311 meters. The feeblest light which has been here meas 

 ured with the standard slit is that by F. W. V. on April 2d at 



wave-length 0^*40 which is put down at 0*13 corresponding to 

 a distance of 36 cm from slit (s 9 ). 



