Telescopes constructed with such object-glasses were examined by the late Dr. Robison and professor Playfair. The focal distance of the object-glass of one of these did not exceed 17 inches, and yet it bore an aperture of 3-1/2 inches. They viewed some single and double stars and some common objects with this telescope; and found, that, in magnifying power, brightness, and distinctness, it was manifestly superior to one of Mr. Dollond of 42 inches focal length. They had most distinct vision of a star, _when using an erecting eye-piece_, which made this telescope magnify more than a 100 times; and they found the field of vision as uniformly distinct as with Dollond’s 42 inch telescope magnifying 46 times; and were led to admire the nice figuring and centering of the very deep eye-glasses which were necessary for this amplification. They saw double stars with a degree of perfection which astonished them. These telescopes, however, have never yet come into general use; and one reason perhaps, is, that they are much more apt to be deranged, than telescopes constructed of object-glasses which are solid. If any species of glass, or other solid transparent substance could be found with the same optical properties, instruments might perhaps be constructed of a larger size, and considerably superior to our best achromatic telescopes.[23] It is said that Mr. Blair, the son of Dr. Blair, some years ago, was engaged in prosecuting his father’s views, but I have not heard any thing respecting the result of his investigations. _Barlow’s refracting telescope with a fluid concave lens._