ON COLOUR PHOTOMETRY. 
443 
Observer II. —H had no idea that he was deficient in colour perception. He is a gentle¬ 
man, aged 74, of an acute and educated mind. He attributed his miscalling colours to 
an ignorance of their names rather than to any deficiency of perception. At first we 
were not sanguine as to what the results of our tests might be, as we thought he might 
be right, having sometimes met people wh ose sole defect is a want of what may be called 
colour education. Our doubts were however very soon dispelled, and the observations 
proved to be among the most interesting which we have to record. He had never in 
his life made a photometrical observation so far as we could learn, so that his test 
thoroughly tried the practicability of our method as to ease of observation. One of 
us however moved the slit for him, and asked him to say “ too light” or “ too dark” 
as to the shadows, and by gently oscillating the slit we had no difficulty in arriving at 
his value of the luminosity. Considering his want of practice the readings were very 
fairly concordant, and can, we believe, be relied on, in which belief we were confirmed 
by comparing his curve with that of B. It will be seen, fig. 5, Plate 24, that, like B, 
his red deficiency curve starts from E, and that his luminosity curve ends at 43’6. His 
blue deficiency is most pronounced. Between C and 43'6 his curve is fuller than B’s, 
as if his perception to red at that part of the spectrum were greater. But it may be 
that with the feeble illumination of that part his readings were not so correct, and they 
were certainly less concordant here than in other parts. We have, however, plotted 
a curve from the means of the readings, and this makes the maximum of the red 
deficiency curve rather nearer 46 than 45 on our scale. 
We append notes of the colours of different parts of the spectrum as he described 
them. 
At 46'6 (D) 
Yellow or green, more green than yellow. 
46 
Orange. 
44 
(1st time) scarlet like sunset. 
(2nd time after several intermediate colours had been shown) dark green. 
54 
Blue. 
49-6 
White, a slight tinge of green. 
49 
Very light green. 
When the two shadows were placed alongside one another as in the photometry he 
described. 
Candle. 
Spectrum. 
49 
Brown. 
Light violet. 
49-6 
Brown. 
Green. 
46-2 
Dark blue. 
Green. 
3 l 2 
