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PROFESSOR FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
Section I. — Qualities of Rays. 
3. Not to incur a charge of vagueness, to which our remarks may be thought liable 
w r hen we speak of light and heat together, I would first notice the difficulty under 
which we labour in this respect. Our instruments are so imperfect that it is difficult 
to say what particular effect we are truly measuring at a given time. The solar rays 
contain light and heat ; and each of these results of radiations varies, first, in respect 
of intensity, and secondly, in respect of quality. We may seem to perceive a change 
in the first of these effects when it is only the second which has varied. Light may 
be more or less brilliant, but it may also be red or yellow. Heat may be more or 
less intense, but it also changes its quality in a manner similar to the effect of color- 
ation for light. A person who could distinguish only yellow rays of light, would feel 
as if plunged in darkness when the red ray of the spectrum was directed on his 
eye. Our instruments for the measurement of heat possess undoubtedly something 
like the quality now supposed to exist in a natural eye. They may indicate, not 
the whole intensity of the incident heat, but only the intensity of that modifi- 
cation of it, to which the measurer applied is exclusively or peculiarly sensitive. 
As we employ a black, or a blue, or a white thermometer to measure the force of the 
solar rays, we shall have indications not only absolutely different, but relatively dif- 
ferent under different circumstances. Interpose, for instance, a plate of glass, and 
though the same reduced quantity of heat falls upon all the three instruments, the 
black thermometer will sink less than the white one would do*. We cannot tell but 
that the atmosphere acts as the plate of glass does, and therefore, that the indication 
of the opacity of the atmosphere in respect to the heating rays is only true as regards 
a certain class of heating rays : nay, it is almost demonstrated that such is the case ; 
and that consequently we must use “ law of extinction,” “ transparency of the at- 
mosphere,” and such terms, with especial and exclusive reference to the class of effects 
which our instrument is capable of measuring. 
4. The direct quantitative measurement of light has not yet been satisfactorily 
accomplished ; and of the indirect methods, some depend upon the faculty of the eye 
in comparing illuminated surfaces, and others upon the thermometric effects which 
the luminous calorific rays produce. We can by no means conclude that these two 
methods, so dissimilar, of estimating the loss of solar light in its transit through the 
atmosphere, ought to give identical results. The one was practised by Bouguer, the 
other by Lambert; and it is to the latter class alone that the experiments to be de- 
scribed in this paper belong : but before detailing them, it may be well to glance 
historically at the more important parts of the problem. 
Section II. — History of the Inquiry. 
5. Sir Isaac Newton, in the third book of his Optics, speaks of the opacity or 
imperfect transparency of bodies as arising from the multiplied reflections of light 
* Powell, Philosophical Transactions, 1S25. 
