214 | Dr. G. J. Burch. [ Mar. 7, 
so long as to be practically never absent. And in such cases there would 
always be, as some observers have maintained, an “interval between the 
absolute and the chromatic images.” When my eyes have not quite lost the 
“dazzle-tints” there is, to me also, such an interval. But when they are 
completely rested there is none. 
Hxperiment 11.—According to the theory of Young and of Helmholtz, each 
portion of the spectrum with the exception of its two ends, excites more than 
one colour sensation. My own experiments on artificial colour blindness 
support this view. The observations recorded in the present paper show that 
the effect of rest in darkness is to increase the sensitiveness of the eye very 
much more to the highly refrangible than to the less refrangible rays. It 
follows that a colour like that of the sodium flame which, to a normal eye, 
excites the red and green sensations in almost equal proportions will, if the 
intensity is greatly reduced and the eye sufficiently rested, excite a larger 
proportion of green. This I have put to the test of experiment. The results 
are interesting as bearing on the precautions necessary in such cases. A 
Bunsen flame in which was a piece of glass tubing containing crystals of 
sodium thiosulphate was placed at such a distance from a paper screen outside 
the dark room, that its light, reflected on to a second piece of white paper 
inside the dark room in the place usually occupied by the spectroscope, was 
barely visible to the perfectly rested eye. Lt appeared whitish-grey even when 
considerably brighter than the minimum visible. The reason of this was evident 
when I removed the second paper reflector and examined the light from the 
first with the spectroscope. In addition to the sodium lines there was the 
spectrum of the Bunsen flame itself with its three bands in the green, the 
blue, and the violet. But in the spectroscope, where these other colours were 
separated from them, the sodium lines appeared pale green when of the minimum 
visible intensity. 
The sensitiveness of the eye to blue increases so greatly after two hours in 
darkness that nothing but spectrum analysis can ensure its absence—and a 
very small trace may suffice to change orange to white. or the same reason 
we cannot take the green of the spectrum at E or 0 as the standard green for 
the minimum visible. We know that the red and the blue sensations over- 
lap there, even if the violet does not reach so far. The colour must look pale 
under feeble illumination owing to the presence of three if not all of the 
constituents of white. The greatest contrasts seem to be obtained, so far as I 
have gone, with light from B for the red, from D for the green, eon F for 
the blue, and from H and K for the violet. 
Owing to this overlapping of the colours, I have not yet satisfactorily 
