Prof. Maxwell on the Theory of Compound Colours. 145 
corresponding to these form a series of submultiples of the retarda- 
tion ; and by counting the bands between two of the fixed lines, whose 
wave-lengths have been determined by Fraunhofer, the wave-lengths 
corresponding to all the bands may be calculated ; and as there are 
a great number of bands, the wave-lengths become known at a great 
many different points. 
In this way the wave-lengths of the colours compared may be 
ascertained, and the results obtained by one observer rendered 
comparable with those obtained by another, with different apparatus. 
A portable apparatus, similar to one exhibited to the British 
Association in 1856, is now being constructed in order to obtain 
observations made by eyes of different qualities, especially those 
whose vision is dichromic. 
PosTscRIPT. 
Account of Experiments on the Spectrum as seen by the Colour-blind. 
The instrument used in these observations was similar to that 
already described. By reflecting the light back through the prisms 
by means of a concave mirror, the instrument is rendered much 
shorter and more portable, while the definition of the spectrum is 
rather improved. The experiments were made by two colour-blind 
observers, one of whom, however, did not obtain sunlight at the 
time of observation. The other obtained results, both with cloud- 
light and sun-light, in the way already described. It appears from 
these observations— 
I. That any two colours of the spectrum, on opposite sides of the 
line “ F,” may be combined in such proportions as to form white. 
II. That all the colours on the more refrangible side of F appear 
to the colour-blind “ blue,”’ and all those on the less refrangible side 
appear to them of another colour, which they generally speak of as 
*‘yellow,”’ though the green at E appears to them as good a repre- 
sentative of that colour as any other part of the spectrum. 
Ii. That the parts of the spectrum from A to E differ only in 
intensity, and notin colour; the light being too faint for good experi- 
ments between A and D, but not distinguishable in colour from E 
reduced to the same intensity. The maximum is about 2 from D 
towards EK. 
IV. Between E and F the colour appears to vary from the pure 
“yellow”? of E to a “neutral tint”? near F, which cannot be distin- 
guished from white when looked at steadily. 
V. At F the blue and the “yellow” element of colour are in equi- 
librium, and at this part of the spectrum the same blindness of the 
central spot of the eye is found in the colour-blind that has been 
already observed in the normal eye, so that the brightness of the 
spectrum appears decidedly less at F than on either side of that line ; 
and when a large portion of the retina is illuminated with the light 
of this part of the spectrum, the limbus luteus appears as a dark 
Spot, moving with the movements of the eye. The observer has not 
yet been able to distinguish Haidinger’s “‘ brushes’ while observing 
polarized light of this colour, in which they are very conspicuous to 
the author. 
Phil. Mag. 8. 4. Vol. 21. No. 183. Feb. 1862. = 
