169 



telescope had been made some time before the Gregorian, and its 

 speculum had in consequence lost somewhat of its oiiginal polish. 

 But notwithstanding this source of disadvantage on the side of the 

 Cassegrainian, 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 principal focus of the object-glass. 



Additional Observations on the Effects of Magnesia in preventing an 

 increased Formation of Uric Acid ; with Remarks on the Influence 

 of Acids upon the Composition of the Urine. By William Thomas 

 Brande, Esq. F.R.S. Prof. Chem. R. I. Communicated by the So- 

 ciety for improving Animal Chemistry. Read June 3, 1813. [Phil. 

 Trans. 1813,/>. 213.] 



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 re- 

 lieving indigestion, occasioned by the use of alkaline remedies, and 

 who thereby fortunately corrected a tendency to form red sand, for 

 which those medicines had been taken ineffectually. 



