Royal Observatory Greenwich
1851 May 5
Touching the license of displacement for an image of the sun: ¨C it is of no consequence what is the fraction of an inch: all must be referred to fractions of the sun¡¯s diameter, and which amounts to the same thing, to the absolute time employed. In one second of time, the image moves through 15¡å, which is one fourth of the smallest estimation of the height of the red mountains and is therefore fairly allowable. If the image could be formed in a fraction of a second, so much the better.
But I question whether if it be correct that the light of the corona = > 1/7 of moon light an image can be formed in so short a time. This I submit to your judgement.
The red mountains would probably shew as black mountains, which would answer perfectly well.
As to the ¡°remaining light of the sky¡±, will you have the kindness to remark that if you are in the center of the shadow there is not a single direct ray of the sun¡¯s light within 80 miles on any side, and that then for a great distance further it is very feeble. I think you need not to be anxious about remaining light, except that which comes from the corona, and which is therefore faint, included in the comparison.
I differ from you in opinion as to the suggestions being too numerous. Already I could wish that one or two more had been added. It would be insane in any observer not to select his class of observations before the time arrives
If the state of my family permit, I think of going to Gothenbourg. There is a three-days¡¯-sickness to be gone through, so that I shall probably have the place to myself.
I am, dear Sir, Faithfully yours
G B Airy
Henry F. Talbot Ere
&c &c &c