In Jan. 1824, at Playford, I was working on the effects of separating the two lenses of an object-glass, and on the kind of eye-piece which would be necessary: also on spherical aberrations and Saturn’s figure. On my quires at Cambridge I was working on the effects of separating the object-glass lenses, with the view of correcting the secondary spectrum: and on Jan. 31st I received some numbers (indices of refraction) from Mr Herschel, and reference to Fraunhofer’s numbers.