TRANSACLIIONS OF SECTION Ae 729 
“Dichromic vision allows a match between any four colours, of which black 
may be one. Thus we may find 64 green + 36 blue =61 black + 39 white, a neutral 
matched by a green-blue. But this is apparently not the most searching test. 
The above match was in fact made by an observer whose vision I have reason to 
think is not truly dichromic, for he was unable to make a match among the four 
colours red, green, blue, black. The nearest approach appeared to be 100 red= 
8 green +7 blue + 85 black, but was pronounced far from satisfactory. An observer 
with dichromic vision, present at the same time, made without difliculty 82 red + 
18 blue = 22 green + 78 black, a bright crimson against a very dark green. 
It would usually be very unsafe to conclude that a colour-blind person is in- 
capable of making a match because he thinks himself so, But, in the present 
" instance, repeated trials led to the same result, while other matches, almost equally 
forced in my estimation, were effected without special difficulty. It looked as 
' though the third colour sensation, presumably red, was defective, but not absolutely 
missing. When a large amount of white was present, matches could be made in 
| spite of considerable differences in the red component, but when red light was 
' nearly isolated its distinctive character became apparent. 
This view of the matter was confirmed by experiments with my colour box, in 
which, by means of double refraction, a mixture of spectral red and green can be 
exhibited in juxtaposition with spectral yellow (‘ Nature,’ Nov. 17,1881). A match 
to normal vision requires, of course, that (by rotation of the nicol) the red and green 
should be mixed in the right proportions; and secondly, that (by adjustment of 
gas) the brightness of the spectral yellow should be brought to the right point. 
An observer whose vision is dichromic does not require the first adjustment; any 
mixture of red and green, or even the red and green unmixed, can be matched against 
the yellow. In the present case, however, although the green could be matched 
satisfactorily against the yellow, the red could not. The construction of the 
“instrument allowed the point to be investigated at which the match began to fail. 
Pure green corresponding to 0, and pure red to 25, the match with yellow began 
to fail when the setting reached about 17. Normal vision required a setting of 
about IHG 
_ Truly dichromic vision may be thus exhibited in a diagram. If we take red, 
green, blue, as angular points of a triangle, there is a point upon the plane which 
represents darkness. Any colours which lie upon a line through this point differ 
‘only in brightness. Maxwell determined the point by comparison of colour-blind 
“matches with his own normal ones. It seems preferable to use the colour-blind 
“matches only, as may be done as follows: From the match between red, green, 
‘blue, and black, the position of black on the diagram may be at once determined, 
and for most purposes would represent darkness sufficiently well. A match be- 
tween white and the principal colours will then fix its position relatively to the 
fundamental points. A line joining black and white is the neutral line; all colours 
‘that lie on one side of it are warm, like yellow; all that lie upon the other side 
are cold. The point representing darkness will lie upon the neutral line and a 
little beyond black. The diagram sketched depends upon the following matches 
D 
oo from an observer, whom Holmgren would call green-blind :— 
Red Green Blue Black White Yellow 
— 82:0 +21°8 —18:0 +782 0 0 
+ 57:2 —100 +4:8 0 + 38:0 0 
0 +96°0 +40 —53:0 — 47:0 0 
—100 0 +50 +787 0 +163 
7. On some New Vacuum Joints and Taps. By W. A. SHENSTONE. 
1890. 38 
