☞ The Economy of the Eyes : precepts for the improvement and preservation of the sight : plain rules which will enable all to judge exactly when, and what spectacles are best calculated for their eyes : ... and an account of the Pancratic Magnifier, etc. : Part II. of Telescopes : being ... the result of thirty years' experiments with fifty-one Telescopes. ... To which are added, an abstract of the practical parts of Sir W. Herschel's writings on Telescopes and double stars, etc : some observations thereon, etc.