176 PROFESSOR WARTMANN ON DALTONISM, 
This table shows, notwithstanding some doubtful appellations, 
the evident confirmation of the results obtained with the solar 
spectrum. The thirty-seven plates of glass exhibit to the eye of 
D*** only four different colours, if we except varieties of 
intensity ; only here there are some changes in the name of the 
orange tints and in that of the red and yellow tints which ap- 
proach the nearest to them. The shades which he has a diffi- 
culty to characterize and which he names sometimes reddish, 
sometimes yellowish, are precisely those which are a mixture in 
different proportions of yellow and of red. 
However accordant these trials were, I thought that it would 
be useful to verify them by others, in which I should employ 
pure tints produced by phenomena of coloured polarization*. 
This method appeared to me more sure, because the glasses I 
had employed might present at different places differences of 
thickness or coloration, sometimes also striz which have an in- 
fluence on a very clear refraction of light, and thus introduce 
into delicate researches chances of error which it was important 
to avoid. 
With this view I made use of an apparatus invented by M. 
de Norrenberg. After having disposed the glasses in a conve- 
nient manner, I placed on the stand a quartz perpendicular 
to the axis 0™-002 in thickness, and on the opening of the gradu- 
ated circle a Nichol’s prism. The observations were made on 
Thursday, December 5th, 1839, in snowy weather, at half-past 
2p.m. The readings took place from 5° to 5°, starting from 
90°, placed at the right of the observer, and with whom we com- 
menced by making the mark coincide. In Table II. I give the 
indications of D*** for the circles right and left as well as my 
own made immediately after. I am indebted to the kindness of 
Mr. Taylor, formerly student of the University of Cambridge, 
for checking the results which I have put down. 
(* Sir John Herschel, in his ‘Treatise on Light,’ has given a similar series 
of experiments made by himself on the eyes of the late Mr. Troughton,—Ep. ] 
