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Document number: 3967
Date: 01 Nov 1839
Recipient: TALBOT William Henry Fox
Author: ROSS Andrew
Collection: British Library, London, Manuscripts - Fox Talbot Collection
Collection number historic: LA39-70
Last updated: 26th April 2010

33 Regent St
1st Novr 1839.


I continued the experiments <1> for obtaining a flat field in the camera upon the principle of giving contrary flexure to the curvature of the image found by the object glass by means of the second lens, which should be similar in its effect to the field glass of our Huyghenian eye piece but owing to the great diameter of the object glass the extreme rays of the oblique pencils cross their axes with such great obliquity that the focal length of the pencils forming the marginal part of the picture is thereby so much shortened that I found it impracticable to produce a flat field even by an immoderately deep lens, also from the great difference in the angles of incidence of the extreme rays of oblique pencils and the different manner that rays of the same pencil in [illegible deletion] planes inclined to each other are presented to a lens. I found the marginal distortion incourageable Upon examining the manner that oblique pencils might be transmitted through a lens with the smallest refractions it is seen that of the extreme rays (in one plane) one would require a convex and the other a concave surface. [illustration] this is balanced at the emergence of with some forms of lenses, and repeated in others but in all the forms where the spherical aberration is corrected (which is necessary to obtain intensity) the great difference exists, but if the pencil is made eccentrical and is incident on one side only of the axis of the lens then the extreme rays require the same form of lens to produce the least refraction

The pencil is rendered eccentrical by a diaphragm [illustration] placed in front of the lens where all the rays cross. By this arrangement the extreme rays of the pencil have but small refractions and do not cross their axes so soon as in the oblique pencil therefore their focus is nearly in the same plane as that of the central pencil more particularly if the diameter and distance of the diaphragm is properly regulated – In this construction the great sacrifice is made of intensity and it remains to be ascertained how far two double achromatic combinations having their contact surface cemented can be applied with regard to form and position to produce a flat field at the same time that the spherical aberration is destroyed still retaining the eccentrical form of pencil – I send you the glass I have experimented with fitted to a common camera box, its effect is very beautiful but evidently wants intensity

I have examined Dagurre’s <2> instrument and find that the flat field is obtained in the manner above described but it appears that no attempt has been made to correct the sphl aberration in the glass.

Mons Giroux <3> has not sent the frames to contain the silver plates – I have written for them and for your use in the mean time I have fitted a board and applied a glass as described in your letter. I have also given you the means of varying the aperture of the optical part. This part screws into the tube and the diaphragms only slide

Your Obedient Servant
Andw Ross

H F Talbot Esq

H F Talbot Esqr


1. He was trying various methods of silvering copper plates for making a daguerreotype-like image, a procedure which WHFT was experimenting with in September 1839.

2. A misspelling of Louis Jacques Mandé Daguerre (1787–1851), French artist, showman & inventor. While complete Daguerreotype kits varied, especially as the process was improved, there is a similarity in the complete early outfits as they would have been issued by Giroux in 1839, shortly after the working process was made public. An outfit would have included Daguerre’s manual, a camera with lens and focussing back, a box for polished plates, an iodising box, a mercury box with spirit lamp and thermometer, an “apparatus” (as the plate holder was called), a box of the necessary chemistry, and some hand tools like buffing sticks and a plate polishing vice. Some early outfits also contain an image correction aid, such as a correcting prism (as is found in the Bemis outfit at the George Eastman House) or a 45 degree mirror (as is found in the Svanberg outfit at the Archives of the University of Uppsala).

3. François-Simon-Alphonse Giroux, of Alphonse Giroux et cie, stationers in Paris. Giroux was related to Louis Jacques Mandé Daguerre's wife and in August 1839 secured the exclusive contract to market daguerreotype cameras and outfits manufactured under Daguerre's supervision. With no optical experience, Giroux turned to Charles Chevalier to make the lenses.

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