204 
NATURE 

[uly 13, 187 

the Urgent, while I occupied a second boat nearer to the stern of 
the ship. He cast the plate as a mariner heaves the lead, and 
by the time it had reached me, it had sunk a considerable depth 
in the water. In all cases the hue of this plate was green, not, 
of course, a pure green, but a mixture of green and blue; and 
when the sea was of the darkest indigo, the green was the most 
vivid and pronounced. I could notice the gradual deepening of 
the colour as the plate sank, but at its greatest depth in indigo 
water the colour was sill a blue green. 
“Other observations confirmed this one. The Uygentis a screw 
steamer, and right over the blades of the screw there was an ori- 
fice called the screw-well, through which you could look from 
the poop down upon the screw. ‘The surface glimmer which so 
pesters the eye was here in a great measure removed. Midway 
down a plank crossed the screw-well from side to side, and on 
this I used to place myself to observe the action of the screw 
underneath. The eye was rendered sensitive by the moderation 
of the lipht ; and still further to remove all disturbing causes, 
Lieutenant Walton had the great kindness to have a sail and tar- 
paulin thrown over the mouth of the well. Underneath this I 
perched myself, and watched the screw. In an indigo sea the 
play of colours was indescribably beautiful, and the contrast be- 
tween the water which had the screw-blades for a background, 
and that which had the bottom of the ocean as a background, 
was extraordinary. ‘Tbe one was of the most brilliant green, the 
other of the most lustrous ultramarine, The surface of the water 
above the screw-blade was always ruffled. Liquid lenses were 
thus formed, by which the coloured light was withdrawn from 
some places and concentrated upon others. The screw-blades in 
this case replaced the plate in the former case, and there were 
Other instances of a similar kind. The hue from an indigo sea 
was always green ata certain depth below the surface. The 
white beliies of the porpoises showed the same hue, varying in 
intensity as the creatures swung to and fro between the surface 
and the deeper water. Ina rough sea the light which had pene- 
trated the suinmit of a wave sometimes reached the eye. A 
beautiful green cap was thus placed upon the wave when the ship 
was in indigo water. 
**But bow is this colour to be connected with the suspended par- 
ticles? Take the dinner-plate which showed so brilliant a green 
when thrown into indigo water. Suppose it to diminish in size 
until it reached an almost microscopic magnitude. It would still 
behave substantially as the larger plate, sending to the eye its 
modicum of green light. If the plate, instead of being a large 
coherent mass, were ground to a powder sufficiently fine, and in 
this condition diffused through the clear sea water, it would send 
green to the eye. In fact, the suspended particles which the 
home examination revealed in green sea water act in all essential 
particulars like the plate, or like the screw-blades, or like the 
foam, or like the bellies of the porpoises. When too gross, or in 
too great quantity, the suspended particles thicken the sea itself 
visibly. But when sufficiently small, but not too small, and when 
sufficiently diffused, they do not sensibly interfere with the limpid 
greenness of the sea itvelf. They then require the stronger and 
more delicate test of the concentrated luminous beam to reveal 
their presence.” 

THE TEMPERATURE OF THE SUN 
ROF. NEWCOMB, in reviewing P. A. Secchi’s work 
on the Sun, shows that if the temperature reached 
10,0c0,000° Cent., as asserted by the author of 
“Le Soleil,” the earth would speedily be reduced to 
vapour. In answer to this objection Pére Secchi urges, 
“that a body may have a very high temperature and yet 
radiate but very little ;” contending that ‘‘a thermometer 
dipped ins:de the solar envelope in contact with the photo- 
sphere,” would indicate the temperature mentioned. He 
adds, “ This high temperature, besides, is really a virtual 
temperature, as it is the amount of radiation received 
from all the transparent strata of the solar envelope, and 
this body at the outer shell must certainly be at a lower 
temperature.” What iniormation is intended to be con- 
veyed by thestatementthat 10,000,000° Cent. ‘“‘isreally a vir- 
tual temperature,” on the ground thatitis the “amount of 
radiation received from all the transparent strata” outside 
of the photosphere, I will not attempt to explain ; but I 

propose to show that a thermometer dipped inside the 
solar envelope in contact with the photosphere, cannot 
possibly indicate the enormous temperature of 10,000,090° 
Cent. assumed by Pére Secchi. The assertion that “a 
body may have a very high temperature and yet radiate 
but very little,” were it correct with reference to the 
photosphere, does not affect the question. It is of no 
consequence whether the sun’s photosphere belongs to the 
class of active or sluggish incandescent radiators imagined 
by the distinguished savan; the temperature of the 
radiant surface, not its capacity to radiate more or less 
copiously, is the problem to be solved. Accordingly the 
following statement is intended to show that the tempera- 
ture of the sun’s photosphere at the point where the 
author of “Le Soleil” supposes his thermometer to be 
applied, cannot much exceed 4,000,000° Fahr. Observa- 
tions conducted in lat. 40° 42’, with an actinometer (a 
drawing of which has been published in Lzg7zneering) 
have enabled me to ascertain, with desirable accuracy, 
the intensity of solar radiation for each degree of the sun's 
zenith distance from 17° to 75°. The atmospheric depth at 
the first mentioned zenith distance being only 0046 
greater than the vertical atmospheric depth, I have demon- 
strated, by prolonging the curve constructed agreeable to 
the observations referred to, that the intensity of solar 
radiation on the ecliptic is 67‘20° Fahr. at the time when 
the earth passes the aphelion, The accompanying table, 
the result of two years of observations, shows the atmo- 
spheric depth and the intensity of solar radiation for each 
degree from the vertical to 75° zenith distance. The 
ratio of diminution of intensity of the radiant heat during 
the passage of the rays through the atmosphere being 
accurately defined by this table, it has been easy to calcu- 
late that the amount of retardation of the radiant heat on 
the ecliptic is 0'207 or 17°64° Fahr. Adding this loss of 
energy to the amount of observed radiant heat, it will be 
found that the intensity of solar radiation at the boundary 
of our atmosphere when the earth passes the aphelion 
corresponds with a thermometric interval of 17°64 + 
67°20 = 84°84° on the Fahrenheit scale. Now, the 
aphelion distance of the earth is 218'1 times greater than 
the radius of the sun’s photosphere ; hence, basing our cal- 
culations on the established truth that the intensities are 
inversely as the areas over which the rays are dispersed, 
we prove that the temperature of the photosphere is 
21817 X 84°84° =4,035,584° Fahr. And if we then add 
the amount of loss of intensity attending the passage of 
the rays through the solar envelope, we establish, with 
absolute certainty, the temperature te which a thermo- 
meter will be subjected if “dipped inside the sular 
envelope in contact with the photcsphere.” 
With reference to the retardation of the rays in 
passing through the solar envelope, we possess practical 
data of such a nature that the solution of the problem is 
by no means mere hypothesis. We know that the density 
of atmospheric air would be reduced to sgyp of the ordi- 
nary density if subjected to a temperature of 4,c09,000° 
Fahr. ; hence, if we assume that the solar envelope con- 
sists chiefly of hydrogen, it may be shown, due allowance 
being made for the superior attraction of the sun’s mass, 
that the density of the terrestrial atmosphere at equal 
depth from the boundary is fully 2,000 times 
greater than that of the solar envelope. Accord- 
ingly, as the sun’s rays lose only 17°6° in passing 
vertically through our cold atmosphere, it may be demon- 
strated that the loss of energy during the passage of the 
rays through a transparent solar envelope 80,000 miles in 
depth from the photosphere, cannot exceed oo! or 40,000° 
Fahr. Let us be careful not to confound this diminution 
of energy with the reduction of temperature consequent 
on the dispersion of the raysas they recede from the pho- 
tosphere during their course through the solar envelope. 
The reduction of temperature attending dispersion, ob- 
viously does not involve any diminution of mechanical 
