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._
Professor Barlow, not many years ago, suggested a new fluid telescope, which is deserving of attention; and, about the year 1829 constructed one of pretty large dimensions. The fluid he employs for this purpose is the _sulphuret of Carbon_, which he found to be a substance which possessed every requisite he could desire. Its index is nearly the same as that of the best flint glass, with a dispersive power more than double. It is perfectly colourless, beautifully transparent, and although very expansible, possesses the same, or very nearly the same optical properties under all circumstances to which it is likely to be exposed in astronomical observations--except perhaps, direct observations on the solar disc, which will probably be found inadmissible. Mr. Barlow first constructed an object-glass with this fluid of 3 inches aperture, with which he could see the small star in Polaris with a power of 46, and with the higher powers several stars which are considered to require a good telescope, for example 70, ρ Ophinchi, 39 Bootis, the quadruple star ε Lyræ, ζ Aquarii, α Herculis, &c. He next constructed a 6 inch object-glass. With this instrument the small star in Polaris is so distinct and brilliant, with a power of 143, that its transit might be taken with the utmost certainty. As the mode of constructing these telescopes is somewhat novel, it may be expedient to enter somewhat into detail.