OF THE RETINA TO LIGHT AND COLOUR. 
183 
These colour lields all liave the same shape (see iig. 12). They do uot cut one 
another, and if we compare the fields of the red and the green with those of the green 
and the blue in the previous table, we shall see that they practically coincide. Thus 
the fields of a red, two greens and a blue, are the same wlien proper luminosities are 
taken for each. Before leaving this table, it is well to point out that the field for D 
is considerably more extended than that of the mixed colours, as are also the fields 
for green and red separately. We may conclude that the intrinsic white light in 
each colour, when added together, is greater than the intrinsic white light in the 
D ray. This points to the fact that the colours of pigments should not give the same 
fields as the spectrum colours with which they approximately match. 
15. Connection hettveen Change of Intensity of Colour and Extent of Field. 
The difterence in extent of field, caused by difterence in illumination, was next 
determined in the horizontal directions. The four rays, red lithium, D, scale No. 417 
in the green, and the blue lithium, were experimented with as fairly representative ol 
