F911. | Colour-sensations by Intermittent Light. 531 
Summary. 
Briefly, the new method of measuring the colour-sensations is as follows :— 
A series of flashes of monochromatic light produce by areal induction a 
condition of colour-blindness in a small area of the field of view, which they 
surround but do not invade. 
This small area is occupied, also in flashes properly timed and _ pro- 
portioned, by the part of the spectrum under examination. The colour 
corresponding to that of the monochromatic flashes is, as it were, wiped 
out of it, and the observer sees the boundaries of the underlying colour- 
sensations as long as the flashes succeed one another at the proper rate. 
A number of small details remain to be settled, but from what I saw 
when I had the model in action in 1908, I am inclined to believe that 
it will supersede all other methods. This apparatus was greatly superior to 
the first form with Thorp’s gratings, figured in my paper on “ Areal Induction.”* 
The greater part of the work was done at University College, Reading, 
in the intervals of teaching, but I was obliged to suspend operations 
altogether in 1905 in consequence of the approaching transference of the, 
College to its new site in the London Road, and it was not until 1908 
that I was able to resume the experiments. I then set up a working model 
of the optical parts and got out measurements for the complete instrument. 
These I was beginning to deal with when my health gave way, and I was 
obliged to limit myself to the work of the department for some time before 
it became necessary for me to give up altogether. 
The materials of the model are now in Oxford, and I hope during the 
summer to superintend the putting of them in working order again. 
Note-—The arrangement of shafting referred to under the head of 
Alternation of Flashes is as follows :—Two steel shafts, one about 4 feet long 
and the other 2 feet long, cross one another at right angles. They are 
connected by bevel gearing. On each is a sliding collar carrying a disc 
about 3 inches in diameter, against which a pair of discs, 9 inches in 
diameter, can be firmly clamped with a nut. The sliding collars can also be 
securely clamped to their respective shafts. In each of the larger discs two 
opposite sectors of 45° are cut away. By rotating one disc in front of the 
other effective apertures of any smaller angle can be obtained, and the 
apparatus being symmetrical remains balanced in all positions. ‘These discs 
revolve in front of the slits of the spectroscopes, which are approximately 
directed at right angles to each other, and are about 3 feet 6 inches apart. 
=< Koy. soc. Proc.,’ vol. 69, p, 127. 
