•364 CAPTAIN, W. DE W. ABNEY AND DR,. T. E. THORPE ON PHOTOMETRIC 
several points along a solar radius extended to the outer limits of visibility ; and 
third, to draw one or more iso-photal lines which should give the contour of the 
corona for varying degrees of illuminating power. The method was to project the 
image of the corona upon the screen of a Bunsen’s photometer in wdiich glass ground 
to slight opacity replaced the greased paper; and instead of one translucent spot 
there was a large number, in order that several hundred samples of coronal light 
taken from different portions of its surface might be observed simultaneously, and 
compared with the standard light by drawing iso-photal contour lines. 
In the apparatus as finally arranged the screen consisted of a piece of perforated 
cardboard covered by a sheet of oiled paper. A number of translucent spots separated 
by opaque interspaces was thus obtained, the spots being sufficiently close together to 
allow the projected image to be seen with but little loss of distinctness. The light 
from the corona was reflected from a heliostat and transmitted through a photographic 
lens of long focus, the ajDerture of which could Ije diminished, if required, by means of 
a cat’s-eye diaphragm, and passed down a dark chamber about 41 feet in length, at 
the end of whicli was a box carrying the screen and standard candles. Within the 
box was a railway, on which ran a wagon mounted on brass wdieels and bearing the 
lighted candle, the distance of which from the screen was recorded on a wooden rod 
placed immediately in front of the observer. The whole apparatus stood on four 
piers, and it was so arranged that any vibration caused by wind should not be 
communicated to the lens producing the coronal image. The focal length of the lens 
was 11’27 metres ; and the diameter of the solar focal image was *104 metre. 
The corona as actually observed was excessively faint, and could only be seen to an 
extent of less than 1' from the Moon’s edge. The coronal light was so feeble that it 
was found impossible to measure its intensity at several points along an extended radius. 
From his observations Professor Langley concludes that the light from the corona 
at 1' from the limb of the Moon was equal to that of the standard candle at a distance 
of 1 metre. From the photometric value of the candle as compared with diffused 
sunlight, Langley further found that the intensity of the coronal light about 1' from 
the limb of the Moon was •0000132 of that of mean sunlight; at 3' from the limb it 
was ’0000000244. Assuming the intensity of mean sunlight to be 500,000 times- 
greater than that of moonliglit, the corona at 1' from the Moon’s limb was six times 
the intrinsic brightness of the Moon ; at 3' it was but one-tenth the intrinsic bright¬ 
ness of the Moon. (Professor S. P. Langley’s Pteport, p. 211, ‘Washington Obser¬ 
vations’ for 1876, Appendix III.) 
The photometric observations made during the 1878 Eclipse have also been discussed 
by Professor W. Harkness, of the United States Naval Observatory (loc. cit., p. 386). 
Combining the observations, he concludes that the total light of the corona was ‘072 
of that of a standard candle at 1 foot distance, or 3'8 times that of the full Moon, or 
'0000069 of that of the Sun. It further ajDpears from the photographs that the 
coronal light varied inversely as the square of the distance from the Sun’s limb. 
