TALBOT3967

33 Regent S^t
1^st Nov^r 1839.

Sir

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 sph^l 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
And^w Ross

H F Talbot Esq^–

H F Talbot Esq^r