
Oct. 5, 1871 | 
NATURE 
451 

high temperaturé of the attenuated matter of which it is 
composed exercises any marked influence on the sun’s 
_ radiant energy, may unquestionably be answered practi- 
cally. An investigation, based on the expedient of con- 
centrating the heat rays of the chromosphere by means of 
a parabolic’ reflector, has been conducted by the writer 
for some time. The method adopted is such that only 
the heat rays, if such there be, from the chromosphere 
and exterior solar envelope, are reflected ; while the rays 
from the photosphere are effectually shut out. Fig. 1 
shows the general arrangement; /’ a’ represents the 
photosphere, and ¢’ / the boundary of the surrounding 
atmosphere; #/Zis a circular screen exactly 10 inches 
in diameter, placed 53'76 inches above the base line ao. 
This distance obviously varies considerably with the 
seasons. Assuming that the investigation takes place 
when the sun subtends an angle of 32’ 1”, the screen £ /, if 
placed at the distance mentioned, will throw a shadow, 
fo, exactly 9°5 inches diameter; hence objects in the 
plane zo placed ‘within fa, will be effectually shut out 
from the rays projected by the photosphere, while they will 
be fully exposed to the rays, if any, emanating from the 
chromosphere and outer strata of the solar envelope. It 
should be observed that, owing to diffraction in con- 
nection with the extreme feebleness of the sun’s rays 
projected from the border, the shadow thrown by the 
screen # / extends considerably beyond the circular area 
defined by fo. Fig. 3 exhibits a /w// stze segment of this 
shadow as it appears round /a, the section coloured black 
in Fig. 2 being a photometric representation of the strength 
of the said shadow from /to a. Special attention is called 
to this photometric representation, as it shows that ob- 
jects placed within the circular area defined by fo are 
absolutely screened from the rays of the photosphere. It 
is evident that a parabolic reflector of proper size placed 
immediately below fo, will concentrate the radiant heat, 
if any, transmitted by the rays /_/ and ¢’ ¢ and theinter- 
mediate rays. Fig. 4 represents a section of the parabolic 
relector which has been employed during the investiga- 
tio. It consists of a solid wrought-iron ring lined with 
silvr on the inside, turned to exact form and highly 
pol'shed. An annular plate 9°5 inches internal diameter, 
is secured to the top of the wrought iron ring to prevent 
efiectually any rays from the photosphere reaching the 
reflector. The prolongation of the rays /’ f/-g’g and 
hn-a'oare shown by dotted lines f, ¢ and 7, 0; also 
the reflected rays directed towards the bulb of the focal 
thermometer, marked respectively /’, 0’ and g’, 7’. The 
investigation not being yet concluded, the following brief 
account is deemed sufficient at present. Turning the 
reflector towards the sun, without applying the screen 
& 1, a narrow zone of dazzling white light is produced on 
the black bulb of the focal thermometer, the mercurial 
column commencing to rise the moment the rays 
strike the reflecting surface. With a perfectly clear 
sky, the column during an experiment on August 29, 
1871, reached 320° Fah. in thirty-five seconds. The 
screen #7 being applied, after cooling the thermo- 
meter, a zone of feeble grey light appeared on the black 
bulb nearly as wide as the oneproduced by the rays from 
the photosphere, but situated somewhat lower. The 
column of the focal thermometer, however, remained 
stationary, excepting the oscillation which always takes 
place when a thermometer is subjected to the influence of 
the currents of air unavoidable in a place exposed to a 
powerful sun. It is proper to remark that owing 
to the stated oscillation, it cannot be positively 
asserted that there was no heating whatever pro- 
duced by the reflection and concentration of the rays 
which formed the zone of grey light adverted to. But the 
recorded oscillations prove absolutely that the heating did 
not exceed 0'5° Fab. Assuming that such a temperature 
was actually produced by the reflected concentrated heat 
emanating from the solar envelope, the following calcula- | 

tion will show that the energy thereby established is too 
insignificant to exercise any appreciable influence on the 
sun’s radiant power. Theoretically, the temperature trans- 
mitted to the bulb of the focal thermometer by the rays 
Jf and o, Fig. 4, is inversely as the foreshortened illumi- 
nated area of the reflector to the zone of light produced 
on the bulb. Obviously these areas bear nearly the same 
relation to each other as the squares of /’ or o’ to the 
square of the radius of the bulb Z. The length of / being 
4'77in., while the radius of the bulb is o'125in. ; calcula- 
tion shows that the temperature transmitted by the ray * 
would be increased 1,456 times if the reflector did not 
absorb any heat. Allowing that 0:72 of the heat is re- 
flected, the augmentation of intensity by concentration 
will amount to 0°72 X 1,456 = 1,048 times the tempera- 
ture transmitted by the rays fand 0. The records of the 
oscillations of the mercurial column during the experi- 
ments show, as stated, that the temperature resulting from 
concentration cannot exceed 0'5°, hence the temperature 
transmitted by the rays emanating from the heated matter 
I 
2 X 1084 
o'00047° Fah. The observations having been made when 
the sun’s zenith distance was 32° 15’, a correction for loss 
occasioned by retardation amounting to 0°26 will, however, 
be necessary. This correction being made, it will be 
found that the heat actually transmitted by the rays from 
the solar envelope during the experiment of August 29, 
did not exceed 0'00059° Fah., a fact which completely dis- 
poses of Secchi’s remarkable assumption that the high 
temperature of the photosphere is owing to the “ radia- 
tion received from all the transparent strata of the solar 
envelope” (see his letter to NATURE, published June 1, 
1871). But we are not discussing the cause ; the degree 
of temperature at the surface of the photosphere is the 
problem to be solved. 
It was stated in the previous article that the radiant 
power of incandescent metals and metals coated with 
lamp-black and maintained at boiling heat, is directly 
proportional to the temperature of the radiator. A series 
of experiments with flames just concluded, proves posi- 
tively that under similar conditions a given area of flame 
of uniform intensity transmits the same temperature as 
incandescent cast-iron. Secchi’s assertion, therefore, 
that the photosphere, if composed of incandescent gases, 
“may have a very high temperature and yet radiate but 
very little,” is wholly untenable. The diminution of in- 
tensity attending the passage of the heat rays from the 
photosphere through the surrounding atmosphere, is the 
only point which can materially affect the question 
of temperature. We have shown that on a given 
area, the quantity of matter {contained in the solar at- 
mosphere cannot greatly exceed that of the terrestrial 
atmosphere; hence the retardation cannot be great. 
True, the depth of the solar envelope is vast compared 
with that of the earth’s atmosphere, but distance fer se 
does not affect the propagation of radiant heat. Admitting, 
however, the retardation to be as the cube root of the 
depth—the ratio observed in the terrestrial atmosphere—--- 
it will be found that the loss of energy produced by re- 
tardation of the heat rays is not important. The solar 
atmosphere being a = 2381 times deeper than the 
of the solar envelope will only amount to 
earth’s atmosphere, the retardation caused by the former 
will be 13°3 times greater than that of the terrestrial at- 
mosphere, which, as we know, diminishes the radiant in- 
tensity 17°64° on the ecliptic. Accordingly we are justified 
in asserting that 13°3 xX 17°64° = 234 6° Fah. will be the 
greatest possible diminution of temperature caused by 
the retarding influence of the matter composing the solar 
envelope. The admission in the previous article, that the 
retardation under consideration might be oor, was based 
on the extreme assumption that the obstruction is directly 
