OF THE RETINA TO LIGHT AND COLOCR. 
185 
Fig. 13. 
With my assistant (VV. B.), these numbers appear to be 4 and ‘2‘5 respectively, 
showing" a consistent variation from my own measures. That there is a diminution in 
the angle of field in an arithmetical progression, as the intensity diminishes in 
geometrical progression, is somewhat strange, and appears to be unaccountable. It 
vill be noticed that the region of the macula lutea has been avoided in these obser¬ 
vations, as it seemed to be useless to attempt any observations on parts of the 
retina which were evidently unsuited for them. 
16. Extent of Field for the Different Rays of the Spectrum, 
It nou became of interest to ascertain the extent of the fields to my own eyes when 
a slit was passed unaltered through the spectrum, for it then became a matter of calcu¬ 
lation to find the intensity (luminosity) of each colour required to give equal horizontal 
fields at any given angular distance from the centre of the retina, and as all fields 
are similar, when one has been measured for any colour, all the others may be con¬ 
structed. The following is a table of three sets of observations. The two first were 
taken with an aperture of *525 inch, with an angular value of 2° 30'. The third was 
taken with an aperture of -086 inch, embracing an angle of 25' only, the temporal 
extent being only observed with it. 
VOL. CXC.—A. 2 B 
