Lacock Abbey,
5 April 1843
Dear Sir
I return your positive Cyanotype. These new processes which you have discovered are very interesting. Which is the most sensitive?
The positive Calotype is a very pleasing method. It is less sensitive than the negative, perhaps 10 times, but in this class of phenomena correct numerical comparisons are impossible. To make a positive image of a piece of lace, or object of that kind, 1 second of sunshine is ample.
In a week or 10 days I am going to Paris, & shall take that opportunity to instruct some French artists in the Calotype of which I believe they have no notion, altho’ published 2 years ago.
This seems to have been a return of Cassinis’s Comet of 1668 – if so, it might have been predicted. The description of that one, to be found in Pingré (vol. 2. p. 22) <1> is extremely like the present one.
The first comet of 1702 was also similar (Pingré p. 37) Compare also that of 1491 & that of 1313. both of which seem to have been similar in physical & external characters to the present one.
| 1491 |  |
177 years | 1668 | |
175 | |
| 1668 | 1843 | |||||
| 1313 | |
178 years | ||||
| 1491 | ||||||
The general problem of determining a parabolic orbit from 3 observations, is well known to present great difficulties, & no rigorous solution is known – But when 2 of the observations are in the Ecliptic, they suffice to determine the Elements, and the problem reduces itself to the following, of remarkable simplicity.
[illustration] Given an angle ABC & a point in it D, to draw the straight line EDF so that it may be of given length. Hence 2 parabolas & no more answer the conditions of the problem.
I have frequently made photographs which were pos. on one side of the paper, & neg. on the other; but I have not had leisure to investigate the cause: any more than that of a variety of [illegible deletion] other remarkable results which are continually occurring –
I remain dear Sir yours very truly
H. F. Talbot
Notes:
1. Alexandre Guy Pingré, Cométagraphie, ou Traité historique et théorique des comètes (Paris: de l’imprimerie Royale, 1783–1784).