To distinct vision it is necessary not only that the visual angle should be sufficient for the powers of the human eye, but that there should be sufficient light also on the object of observation. In microscopic observations, the enlargement of the angle of vision may be more indulged, because auxiliary light may be concentrated on the object by concave mirrors. But in the case of the heavenly bodies, we can have no such aid. The moon, for example, receives from the sun but a fixed quantity of light. In proportion as you magnify her surface, you spread that fixed quantity over a greater space, dilute it more, and render the object more dim. If you increase her magnitude infinitely, you dim her face infinitely also, and she becomes invisible. When under total eclipse, all the direct rays of the sun being intercepted, she is seen but faintly, and would not be seen at all but for the refraction of the solar rays in their passage through our atmosphere. In a night of extreme darkness, a house or a mountain is not seen, as not having light enough to impress the limited sensibility of our eye. I do suppose in fact that Herschel has availed himself of the properties of the parabolic mirror to the point beyond which its effect would be countervailed by the diminution of light on the object. I barely suggest this element, not presented to view in your letter, as one which must enter into the estimate of the improved telescope you propose. You will receive from the professional mathematicians whom you have consulted, remarks more elaborate and profound, and must be so good as to accept mine merely as testimonies of my respect. TO MR. GEORGE F. HOPKINS. MONTICELLO, September 5, 1822.