THE THEOET OF COMPOUND COLOHES. 
79 
This instrument gives a spectrum in which the lines are very distinct, and the length 
of the spectrum from A to H is 3'6 inches. The outside measure of the box is 3 feet 
6 inches, by 11 inches by 4 inches, and it can be carried about, and set up in any posi- 
tion, without readjustment. It Avas made by Messrs. Smith and Ramage of Aberdeen. 
In obtaining observations from colour-blind persons, two slits only are required to 
produce a mixture chromatically equivalent to white ; and at one point of the spectrum 
the colour of the pure rays appears identical with white. This point is near the line F, 
a little on the less refrangible side. From this point to the more refrangible end of the 
spectrum appears to them “ blue.” The colours on the less refrangible side appear to 
them all of the same quality, but of different degrees of brightness ; and when any of 
them are made sufficiently bright, they are called “ yellow.” It is convenient to use the 
term “ yellow ” in speaking of the colours from red to green inclusive, since it Avill be 
found that a dichromic person in speaking of red, green, orange, and brown, refers to 
different degrees of brightness or purity of a single colour, and not to different colours 
perceh’ed by him. This colour we may agree to call “ yellow,” though it is not pro- 
bable that the sensation of it is like that of yellow as perceived by us. 
Of the three standard colours which I formerly assumed, the red appears to them 
•' yellow,” but so feeble that there is not enough in the Avhole red division of the spec- 
trum to form an equivalent to make up the standard white. The green at E appears a 
good “yellow,” and the blue at f from F towards G appears a good “blue.” I have 
therefore taken these as standard colours for reducing dichromic observations. The three 
standard colours Avill be referred to as (104), (88), and (68), these being the positions of 
the red, green, and blue on the scale of the new instrument. 
Mr. James Simpsox, formerly student of Natural Philosophy in my class, has furnished 
me Avith thii'ty-thi-ee obsenations taken in good sunlight. Ten of these Avere between 
the two standard colours, and give the following result : — 
33-7(88)+33-I(68)=W (I.) 
The mean errors of these observations were as follows : — 
Error of (88) =2 -5; of(68) = 2-3; of (88) + (68) = 4-8 ; of (88)-(68) = l-3. 
The fact that the mean error of the sum Avas so much greater than the mean error of 
the chfference, indicates that in this case, as in all others that I have examined, obsein^a- 
tions of equality of tint can be depended on much more than observations of equality of 
illumination or brightness. 
From six obseiwations of my oAvn, made at the same time, I have deduced the 
“ trichromic ” equation 
22-6(I04)+26(88) + 37-4(68)=W (2.) 
If Ave suppose that the light Avhich reached the organ of vision was the same in both 
cases, we may combine these equations by subtraction, and so find 
22-6(I04)-7-7(88)+4-3(68)=D, 
M 2 
( 3 .) 
