Mr. Airy on the Total Solar Eclipse of July 8, 1842. 393 
object seen in the telescope with a near one seen without it). 
This power was found very convenient. I had intended occasionally 
to use a higher power, but was prevented from doing so by the fol- 
lowing circumstance. The dark glasses adapted to the higher 
powers were made a little darker than was necessary at Greenwich; 
not, however, so dark as to prevent the most delicate observation. 
But, in consequence of the cloudiness of the day of the eclipse, the 
sun was, for the most part, so faint, that he could scarcely be seen 
when these higher powers, with their corresponding dark glasses, 
were employed; and as the glass for the lowest power was necessa- 
rily still darker, it was useless to attempt to combine the eye-pieces 
for the higher powers with the dark glass for the lowest power. I 
was therefore compelled to lay aside the higher powers. It is cer- 
tain, however, that the power which I used was sufficient for the 
nicest observations which the state of the air permitted, as it showed 
very well the atmospheric undulations on the limbs of the sun and 
moon ; and nothing smaller, of course, could be seen with certainty. 
The dark glass which was used on the power actually employed 
was a combination of a purple and a green glass. It gave to the 
sun’s disc a faint yellow greenish tinge. 
As the eclipse advanced (the sun continuing unclouded), I ob- 
served a circumstance which I have remarked in every solar eclipse 
that I have seen. It is, that the limb of the moon is very much 
more sharply defined than the limb of the sun. This is clearly 
owing to the difference of intensity of light on different parts of the 
sun’s disc, the intensity near the centre of the disc being much 
greater than that near the limb, and the degradation very near the 
limb, though rapid, being gradual. I speak of this as a fact of which 
I have not the smallest doubt; having long ago observed it in my 
daily practice as an observer with the transit instrument and the 
circle ; and having also frequently remarked it as an experimenter 
when I have thrown the image of a portion of the sun’s disc upon a 
small screen, in which case I have always been able to determine 
whether the limb was approaching to the edge of the fully illumi- 
nated screen, simply by the change of the intensity of illumination. 
And I allude specially to this fact at present, first, because in con- 
templating the probable phenomena of the eclipse it had been a 
particular subject of conversation between Professor Forbes and 
myself; secondly, because it may perhaps assist to explain a very 
strange observation which I shall shortly have to mention. 
I had carried with me a wax-taper in a lantern, which I lighted 
about the time of the commencement of the eclipse. Whenever I 
looked round over the country, I also looked at the flame of the 
taper. I cannot, however, say that I remarked any peculiarity in 
the colour either of terrestrial objects or of the candle-flame. ‘The 
flame, as the eclipse advanced, appeared much brighter (and must 
have been visible to a great distance at the totality), and its colour 
seemed somewhat redder : but I believe that this change takes place 
in just the same degree when the general light is diminished from any 
other cause. ‘The surrounding objects did not receive the greenish 
Phil. Mag. 8. 3. Vol. 22. No.146. May 1843. 2D 
