190 Mr. Powell's experimental enquiry into the nature of 
the general principle of the following experiments is this : 
taking different luminous hot bodies, to expose to their influ- 
ence two thermometers presenting, one, a smooth black sur- 
face, the other an absorptive white one : thus obtaining the 
ratio of the total direct effect on the two, we may compare it 
with the ratio similarly observed, when a transparent screen 
is interposed. 
(7.) This principle of experimenting was applied with one 
or two variations : and though in the abstract sufficiently 
simple, it will in practice require an attention to several con- 
siderations. I shall therefore proceed in the first instance to 
the detail of the different particulars ; then give the results of 
the experiments in a tabular form ; and lastly, recapitulate the 
conclusions, and make a few general remarks. 
I. (8.) In the following set of experiments two common 
thermometers were employed. The diameters of their bulbs 
were, thermometer A, 0.6 inch, ; B, 0.55. A, was coated 
with a wash of chalk and water, and B, with indian ink. 
In order to compare the effects to be observed with those 
of simple radiant heat, I ascertained the ratio of the effects of 
the latter on the two bulbs thus coated, by a few preliminary 
trials ; and found it to be very nearly one of equality, or 
perhaps the effect of the white rather greater than that of 1 he 
black. 
The two thermometers were graduated to quarters of cen- 
tigrade degrees ; and were both fixed on one mounting with 
their bulbs detached about one inch from its lowest part, and 
at the distance of about three-quarters of an inch from each 
other. 
(9.) In the 2d set of experiments they were fixed into the 
