As you were so good as to suggest a mode of procuring from M. Fraunhofer <1> such glasses &c as I might wish to obtain, for optical & astronomical purposes, through the medium of the Bavarian Ambassador (with whom I have not the honour to be personally acquainted) and to offer your assistance for that purpose – Allow me to request your good offices in obtaining for me the following list of instruments – the money for which I will take care to pay in any way & at any time Mr Fraunhofer will suggest.
1st. Four isosceles right-angled prisms (ie having their angles 90°, 45°, and 45°) of the purest flint glass, absolutely free from veins, and polished to very true planes on all their three faces. They are to serve for the small reflectors of Newtonian Telescopes by internal reflection. – Dimensions as follows.
Sides of the Prism Each a square, of 1¼ English inch in length & breadth.
Hypothenuse an oblong of in 1·7675 (= 1¼ x √2) in length and 1½ inch in breadth. NB. It is essential that they should be exactly isosceles.
& perfectly free from veins
2dly. Three Achromatic object-glasses in their cells in which the Spherical aberration is rigorously destroyed, 1½ inch diameter, and 12 inches focal length. They are for Collimators. <2>
3dly. A set of double convex lenses of the following focal lengths. 1 inch, 3/4 inch, 1/2 inch, 1/3 inch, 1/4 inch, 1/5 inch.
4thly. A set of plano-concave lenses of the same focal lengths.
5thly. Two double convex lenses of 50 feet focal length and 3 inches diameter
6th. One isosceles right angled prism similar to No 1 but having its sides 1½ inch instead of 1¼
I remain Dear Sir Yours truly
Devonshire Street <3>
H. F. Talbot Esqr
31 Sackville Street
1. Joseph von Fraunhofer (1787–1826), optician, Munich. This must have been discussed in person when the two men met on 3 January - See Doc. No: 01349. In February, WHFT placed an order for optical equipment from Fraunhofer. [See Doc. No: 01371].
2. At this time a collimator would take the form of a small fixed telecscope with cross-wires at its focus, used for adjusting the line of sight or the line of the optical axis (the line of collimation) of an astronomical instrument.