Index-map of stars based on Messier, by William Lassell Sketch mapping stars about Messier 17, 18, 13 with scale given for a field of 228 eyepiece, in Lassell's hand. Stars are numbered. No correspondence between Sir John Herschel and William Lassell is known, but from a letter to George Biddell Airy, we know that Lassell wrote to Herschel in 1848 to report his discovery of the 8th satellite of Saturn. 1840s