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near the fact. Professor Langley applied well-known photometric methods 
to the problem. By attaching a circle of cardboard to the equatorial tele- 
scope, a solar image is received on the board, plainly showing spots, 
penumbrse, &c., if the image be one foot in diameter. From holes in this 
cardboard pencils of rays issue, which being caught on a screen give a 
second series of images. If these images are caught upon separate mirrors, 
instead of a screen, their relative light can be made the subject of com- 
parison with that of a disc of flame from Bunsen’s apparatus, and thereby 
their relative intensity determined. Between each aperture and its respec- 
tive mirror a lens was interposed which concentrated the pencil of rays. 
By suitable additions this apparatus can be converted to a Rumford photo- 
meter, and in this form it proved most available in Professor Langley’s 
hands. He found a value for the brilliancy of the umbra in sun-spots 
considerably higher than that hitherto computed. The blackest umbra, he 
finds, is between 5,000 and 10,000 times as bright as the full moon. The 
light of the sun is absorbed by its atmosphere not in the same, but in a 
greater proportion than its heat. A long series of experiments shows that 
not much more or less than one-half of the radiant heat of the sun is 
absorbed or suffers internal reflection by the atmosphere of the sun itself. 
Observations indicate that this atmosphere is (speaking comparatively) 
extremely thin : Professor Langley is inclined to regard it as identical with 
the te reversing layer ” observed by Dr. Young, of Dartmouth, at the base of 
the chromosphere, though the chromospheric shadow should perhaps be 
taken into the account. The importance of a study of this absorbent at- 
mosphere becomes evident if we admit that the greater part of the 500° 
which separate the temperature of the temperate zone from absolute zero is 
principally due to the sun’s radiation. To this atmosphere new matter is 
constantly being added and taken away by the continual changes of the 
interior surface. Any alteration in the capacity for absorption — say a 
difference of 25 per cent., which could hardly be recognised by observation 
—would alter the temperature of our globe by 100°. The existence of life 
on the earth is clearly dependent on the constancy of the depth and absorp- 
tion of this solar envelope. Hitherto we have chiefly confined calculations 
to the diminution of solar heat by contraction of the sun’s mass — an opera- 
tion likely to go on with great uniformity. But here is an element of far 
more rapid variation. If changes in the depth of this solar envelope are 
cyclical, they would be accompanied by cyclical alterations of the earth’s 
temperature. This may serve alike to explain the characteristics of variable 
stars and the vast secular changes on earth indicated by geology. If the law 
of alterations in that envelope can be ascertained, new light may be shed on 
the history of the globe and the near future of life upon it. 
Changes in Indian Observatories . — The 11 Astronomical Register ” (August) 
understands that Government will, in all probability, sanction the transfer 
to Simla of the magnificent set of astronomical instruments now in charge 
of Colonel Tennant at Roorkee ; and also the establishment at the former 
station of a permanent observatory, under the direction of the gentleman 
whose observations of the transit of Yenus at Roorkee elicited so much 
admiration. 
Diameter of the Sun , — From a discussion of the Greenwich observations, 
