and the Cause of Colour. 367 © 
confined to that part in the spectrum which is visible; each ex- 
tends from its own side, in quantity gradually lessening, through 
all the visible rays; so that, as far as they are united, vision is 
produced; and when they are separated, it ceases. Nor does it 
appear ever to have been satisfactorily proved, that either the 
calorific or chemical rays have been entirely separated from each 
other, while the power of producing vision continued. And no 
effects are produced by light, except those relating to vision, 
which may not reasonably be attributed to one or both of the 
invisible rays. These circumstances would naturally suggest the 
possibility, that vision may be caused by peculiar combinations 
of those rays which when separate, or so nearly so as to have 
the same effect to our senses, are invisible*. 
Dr. Herschel’s experiments on the subject are by no means 
conclusive against this hypothesis. He has separated some of 
the rays from the different coloured divisions of the spectrum ; 
and has found, that the diminution in the power of rendering 
objects visible, was in a different proportion, from the loss of 
power to raise the thermometer. But without here inquiring 
whether the calorific rays are the same as caloric, it is evident, 
that if the degree of light depend, not only upon the quantity, but 
also upon the proportionate union of the calorific and chemical 
rays, the excess of either might be lessened, without reducing 
the power of producing vision, proportionately with the absolute 
quantity of the rays abstracted. 
. Another 
* This opinion was, I believe, first before maintained by Dr. Hunter, in 
an inaugural dissertation at Edinburgh, in the year 1808; but which it has 
not been my good fortune to be able to meet with. 
+ The labour and attention bestowed by Dr. Herschel on the examina- 
tion of the effect of the different coloured light, on the thermometer and on 
vision, was very great ; and had it been possible to have obtained satisfactory 
results, they could not have been better earned. But the sources of inac- 
curacy in such a course of experiments are so numberless, that no depend- 
ance can be placed upon them. ‘The careful: mention of the mode of ex- 
periment has, however, rendered the discrepancies too apparent to mislead. 
Thus, it is probable that the colour black, frequently arises from an al- 
most exclusive attraction for the calorific ray; certainly it prevents the 
transmission of rays in their coloured state. Yet in the first experiments, 
which were intended to ascertain the effect of the coloured rays of the spec- 
trum upon the thermometer, its bulb was blackened. 
In the other experiments, innumerable causes of error are apparent. The 
temperature of the glass through which the rays were passed is unnoticed ; 
yet perhaps it may be gathered from the experiments, that as that increased, 
the rays were transmitted with a rapidity equal to that with which they 
pass through the air alone. The cause ef the opacity of glass distinct from 
its colour is unknown, and incapable probably of being at present, ascer- 
tained; glass made of the same matcrials not always vitrifying with the 
Same transparency. Nor, possibly, is the human eye capable of that ob- 
Seryation which will enable it correctly to mark a difference so small as 
1-10,000dth 
