OF THE RETINA TO LIGHT AND COLOUR. 
177 
with pig-tnent coloiirs, which, from their nature, are impure. In the ordinary methods 
pursued tlie knowledge that is gained is slight compared with the trouble involved, 
and the colours selected have been based on the assumption that Bering’s theory of 
colour vision is one that has been thoroughly established; experiments from whicli 
a great deal more may be learnt have been neglected or overlooked. I refer especially 
to the measurements of colour fields, where the colours used are pure spectrum 
colours. In these the colour may be isolated and viewed against a black background, 
as for instance, by throwing a spot of light of any desired colour on a white 
surface in a dark room. This is a very different condition to that which obtains 
in the ordinary procedure, wlien the retina receives not only the colour of the 
pigment but also is illuminated by extraneous light. In the experiments to be 
described, two kinds of perimeters w^ere employed. One was the ordinary form but 
modified for use in a dark room. The arc, which subtended a semicircle, was internally 
coated with white, on which degrees were marked at the boundaries, and just below 
the eye was a small mirror on a ball and socket joint, which, by means of an arm, 
would cause a beam of light falling on it to be cast in any direction desired. Thus 
it could be caused to travel along the arc, which might be placed at any angle with 
the vertical. The light employed was that coming from the spectrum of an electric 
arc light, the crater of the positive pole being the source from which the spectrum 
was formed. The colour patch apparatus was employed to get a surface of monochro¬ 
matic light, as described in Part II. of “ Colour Photometry.” A spot of light of any 
desired form or size was obtained by the plan described in my recent paper in the 
‘Proceedings of the Royal Society ’ “On the Formation of Monochromatic Images.” 
The light issuing from the slit in the spectrum could be altered in intensity (1) by 
closing or opening that slit, (2) by placing the annulus already described in front 
of it, (3) by closing the slit of the collimator, (4) by using rotating sectors in front 
of either slit. 
In the second form of perimeter a hollow white hemisphere made of “ papier 
m4che ” was employed. The centre of the surface was pierced with a circular aper¬ 
ture some 1^ inches in diameter. This aperture was closed by a doubly-ground 
glass, and outside the shell apertures of any desired shape or dimensions could be 
placed in contact with the ground glass. The colour patch apparatus was caused to 
throw the patch of colour on to the ground glass, and when the last was removed the 
patch of white that the combining lens cast when the whole spectrum was uncovered 
fell upon the eye when placed at the centre of the hemisphere, thus insuring that 
every ray was equally received on the pupil when the ground glass was again inter¬ 
posed. It may be stated here, once for all, that when light falling on the ground 
glass was measured, by placing a white card in its place and balancing it with an 
amyl-acetate lamp, it was found that the brightness of the ground glass as seen from 
the centre of the hemisphere was within a very small fraction, twelve times that 
which was reflected from the white card. 
2 A 
VOL. CXC.—A. 
