The great advantage which has attended Mr. Dollond’s ingenious application of the negative achromatic lens to the micrometer eyepiece, seems to make it desirable that the principles on which that lens is constructed, and its general application, should be more fully illustrated than is done in the short extract made from my letter to Mr. Dollond, and given by him in his recent paper in the Philosophical Transactions. In my original fluid telescope, the negative lens was employed for the double purpose of lengthening out the focus and correcting the colour of the front lens; and the great advantage of the lengthening principle was manifested by the high penetrating power of the instrument in the centre of the field. Unfortunately, however, the perfect part of this was very limited, so that when Mr. Dollond constructed the second telescope for the Royal Society, I gave up this advantage for the sake of enlarging the held; but I found that by this means much of the penetrating power of the former telescope was lost; for although I had the same aperture, many small stars which were before very perspicuous were in this instrument seen only with difficulty and under advantageous circumstances of weather, absence of moonlight, &c.
XII. On the principle of construction and general application of the negative achromatic lens to telescopes and eyepieces every description