ON COLOUR PHOTOMETRY. 
537 
§ XLVII .—The Limit of Colour Vision. 
It is well-known that as light of any colour becomes enfeebled the eye fails to see 
colour, though it can recognise the presence of light. From a physical as well as 
from a physiological point of view, it appeared to be of interest to ascertain the amount 
of illumination of a screen at which all appreciation of colour in the different rays of the 
spectrum disappeared, leaving a sensation of what, for want of a better word, we may 
call grey light. In order to ascertain this, an apparatus (fig. 36) was devised as a 
Fig. 35. 
Curves stowing the luminosity of the spectrum when measured (1) with the fovea centralis, (2) with the 
area of the yellow spot or macula lutea, (3) with the retina 10° from the fovea centralis and outside the 
macula lutea. 
supplement to that already described, by which a white light of very low intensity 
could be compared with the spectrum colours. 
A.t one end of a box, shown in plan, is an eye-piece E. The other end has at its 
centre a patch S, 1^ inches square, whitened with zinc oxide, the rest of the inside of 
the box being blackened. The monochromatic beam a coming from the spectrum 
through the side slit, and the reference beam b, are reflected by jflain glass mirrors 
M X M % to apertures in opposite sides of the box, and from just inside these apertures, 
by right-angled prisms P 1 P 2 so as to fall on and cover S. Rods R X R 2 are inserted in 
the box in the paths of the beams so that they illuminate opposite halves of S . 
MDCCCXCII.—A. 3 Z 
