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The author having remarked the performance of a Cassegrainian telescope, made by a self-taught artist at Ipswich, to be superior to what he believes is usually expected from telescopes of this construction, has been led to make a series of experiments on the comparative illumination given by the Cassegrainian compared with that obtained by the Gregorian construction. For though the Cassegrainian form has been considered merely as the Gregorian disguised, and has been rarely adopted, in consequence of its inverting objects, a superior power of illumination, if correctly ascertained to exist, may prove a valuable property, in addition to its advantage of being considerably shorter than the Gregorian. In the telescope first compared by Major Kater, the specula were cast at the same time, in the same metal, and to the-same pattern. The magnifying powers of the two instruments were ascertained by experiment to be very nearly equal, but with a small excess on the side of the Cassegrainian. The two telescopes were placed side by side, and pointed to the same object, which was a printed card, at the distance of fifty yards ; and as the brightness, as seen in the Cassegrainian, was far superior, its aperture was first reduced by a ring of pasteboard, and then gradually enlarged till the card appeared equally bright through both telescopes. After the respective areas of aperture in each telescope had been measured, with due allowance for the light obstructed in each by the small mirror, that of the Cassegrainian was found to be to the Gregorian as 46 to 108, or 3 to 7 nearly.

On the light of the Cassegrainian telescope, compared with that of the Gregorian