Page:Proceedings of the Royal Society of London Vol 1.djvu/489

Rh telescope had been made some time before the Gregorian, and its speculum had in consequence lost somewhat of its original polish. But notwithstanding this source of disadvantage on the side of the Casscgrainian, a corresponding superiority again appeared in its power of illumination.

The areas of aperture were in this instance as 79 to 110; but as the magnifying powers were not equal but in the ratio of 108 to 182. it was necessary to make further allowance, in proportion to the squares of these numbers; so that the illuminating powers were found to be nearly as 3 to 2.

From the mean of these experiments, and from consideration of all circumstances, the author conceives that the relative superiority of the Cassegrainian may be stated to be as 60 to 33, or 20 to 11.

With respect to the probable cause of the difference thus observed, Major Kater conjectures that it may possibly depend on the mutual interference of rays meeting in the same point, which it is possible _may be in great measure dissipated when received by the small spe- culum in the Gregorian, after crossing in the principal focus; while on the contrary, in the Cassegrainian, the loss of light from this source is avoided, since the small speculum in that construction re- ceives the rays before they arrive at the focus, and before they be- come sufﬁciently concentrated to interfere with each other’s motion.

This conjecture. it is observed, derives additional support from a circumstance that has been observed with respect to refracting te- lescopes; namely, that in a comparison between the simple astro- nomical telescope and a Galilean of equal aperture and power, the satellites and belts of Jupiter may be seen much more distinctly in the latter, where the rays are received by a concave lens before their intersection in the primipal focus of the object-glass.

This communication consists of two parts : the ﬁrst of which is a conﬁrmation of the beneﬁcial effects of magnesia in preventing the deposit of uric acid from the urine of persons subject to the formation of a redundancy of that ingredient; and the second part relates to the trial of acid remedies in disorders of an opposite nature, where the urine is found to deposit either the ammoniacal phosphate of magnesia or phosphate of lime.

The ﬁrst section contains two cases; the ﬁrst of a gentleman who was accidentally induced to employ magnesia for the purpose of relieving indigestion, occasioned by the use of alkaline remedies, and who thereby fortunately corrected a tendency to form red sand, for which those medicines had bcen taken ineffectually.