4² × 1,680,000
-------------- = 2,986,000° F.
3

The writer has established the correctness of the assumption that "the
temperature is as the density of the rays," by showing practically that
the _diminution_ of solar temperature (for corresponding zenith distances)
when the earth is in aphelion corresponds with the increased diffusion of
the rays consequent on increased distance from the sun. This practical
demonstration, however, has been questioned on the insufficient ground
that "the eccentricity of the earth's orbit is too small and the
temperature produced by solar radiation too low" to furnish a safe basis
for computations of solar temperature.

In order to meet the objection that the diffusion of the rays in aphelion
do not differ sufficiently, the solar pyrometer has been so arranged that
the density, _i. e._, the diffusion of the reflected rays, can be changed
from a ratio of 1 in 5,040 to that of 1 in 10,241. This has been effected
by employing heaters respectively 10 inches and 20 inches in diameter.
With reference to the "low" solar temperature pointed out, it will be
perceived that the adopted expedient of increasing the density of the rays
without raising the temperature by _converging_ radiation, removes the
objection urged.

Agreeably to the dimensions already specified, the area of the 10-inch
heater acted upon by the reflected solar rays is 331.65 square inches, the
area of the 20-inch heater being 673.9 square inches. The section of the
annular sunbeam whose direct rays act upon the polygonal reflector is
3,130 square inches, as before stated.