120 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
patch apparatus, a spot of light having a diameter of 12 mm. is formed on the screen. 
At the middle, K, of the screen are pierced two pinholes, one vertically over the other 
and 1 inch apart. Behind these holes is placed a lamp, L, and a piece of red glass, N. 
These holes provide fixation points for the observer, who is placed at P at a distance 
of 1 metre from the screen. The wheel, G, is driven by an electric motor at such a 
speed that the spot makes one complete to-and-fro vibration in 1'4 seconds, the 
amplitude of the motion being so great that the spot passes completely off the screen 
at each end of its travel. In its passage across the screen the spot passes half-way 
between the pinholes. The slit, C, is opened by means of a micrometer screw so that 
the brightness of the spot of light can be adjusted. If the slit is placed at 
SSN 40 (52 7/u/u) and the spot is bright, an observer of class I., who keeps his eye 
H 
Fig. 23. 
K 
L 
J 
fixed half-way between the two fixation spots, sees a band of light across the screen 
each time the spot crosses. This band of light appears dark green at the centre and 
a much lighter green and brighter at the sides. As the intensity of the light is 
reduced the centre becomes a darker and duller green while the sides remain bright 
but lose their colour. Finally an intensity is reached when no light at all is seen at 
the centre although the sides are quite bright. It is interesting to notice how very 
well marked is the centre area over which the sensation of light is not perceived, this 
area corresponding of course to the rod-free region of the retina. 
If the same series of changes is repeated with an observer of class II., the results 
obtained are quite different. At fairly high intensities there seems little difference 
between the centre and the sides in colour, though the centre generally seems slightly 
