Letter, from Thomas Davidson to Sir John Herschel, dated at Weardale, Co. Durham Autograph letter signed by sender. Many years ago he read a paper to the Royal Scottish Society of Arts on the merits of John Frederick William Herschel's telescope compared with that of William Parsons (3rd Earl of Rosse); this paper has never been printed. Gives details of his own optical work. 6 February 1862