Letter from G B [George Biddell] Airy, 30 Woburn Place, to Captain [Henry] Kater He has considered the eyepiece that Kater mentioned - two glass hemispheres with the convex sides in contact - giving his opinion on its performance, which he considered in his paper 'On the spherical aberration of eyepieces'. He thinks that a better eyepiece might be made by arranging two similar lenses of equal focal length, radii 6 to 1, with the more convex surface in contact, giving a sketch to illustrate this. If Kater is disposed to try different eyepieces, Airy would be glad to hear the result of such a combination. 13 April 1828