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Cassegrainian Telescope.

Gregorian Telescope. Diameter of the circular Diameter of the large mir- Inch. opening in the paste- Inch. ror

3, 95 board

3,50 Diameter of the back of Diameter of the back of

the small mirror 1,375 the small mirror 0, 95 Length of the arm 1,063 Length of the arm 1, 50 Thickness of the arm 0, 20 Thickness of the arm 0,175

Length of a bar containing
the adjustment

0, 70 Its width

0, 15 Diameter of three semi

circles used as rests for
the great mirror

0,375 From the above measures the following calculations were made. Cassegrainian Telescope.

Gregorian Telescope. Inch. Area of the circular open- Inch. Area of the large mirror 12, 254

ing in the paste-board 9,621 Area of the back of the Area of the back of the

small mirror to be desmall mirror to be de


0,709 ducted

Area of the arm to be de- Area of the arm to be de-


-1,698 Area of the bar contain-

ing the adjustment to
be deducted

Area of the three semi-

circles to be deduct-

1,243 Area of the portion of the Area of the portion of the

mirror exposed to the mirror exposed to the
7,923 light






The magnifying power having been determined (by experiment) to be 188 in the Cassegrainian, and 182 in the Gregorian, the expression for the relative quantity of light becomes


or as 332 to 224, being nearly as 3 to 2. 182 188

In the first experiment, the advantages of polish were, perhaps, on the side of the Cassegrainian telescope; in the last, they were much in favour of the Gregorian; a mean therefore of both results may probably be considered as approaching the truth, and the light of a telescope of the Cassegrainian construction, may be taken, to that of a Gregorian of the same aperture and power, as about 60 to 33. .

A fact so new, naturally leads the mind to hazard a conjecture as to the cause. In the Gregorian telescope a column of light from a point of the object, is received on the large mirror, and reflected in a cone of rays, the vertex of which is its focus, where an image is formed. Here all these rays meet in a single point, and crossing each other, fall on the small concave mirror whence they are again reflected, and form another image near the eye. Now, if light be supposed

. to consist of particles of matter, is it not possible that these particles, crossing in the same point, may interfere with each other? or, when thus forced within a certain distance of each other, may not a power of repulsion exist, which would occasion many of them to be dissipated ? In the Cassegrainian telescope the

rays reflected from the great mirror are received by the small convex mirror before they arrive at their focus, and are consequently reflected back without having crossed as in the Gregorian. The conclusion then seems to be, that wherever an image is formed, much light is lost, and this conMDCCCXIII.



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clusion perhaps derives additional force from a circumstance noticed in most elementary works on optics, viz. that the satellites of Jupiter and his belts, may be distinctly seen with a Galilean telescope, whilst with an astronomical telescope of an equal aperture and power, they remain invisible.

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XXVI. 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 Society for improving Animal Chemistry.

Read June 3, 1819.

In a paper which I had the honour of laying before this Society, about three years ago, and which is published in the Philosophical Transactions*, some cases are related, illustrating the effects of magnesia in preventing an increased formation of uric acid, and some experiments are detailed, instituted with a view to discover its mode of action.

Since that period many opportunities have occurred, both to Sir EVERARD HOME and myself, of confirming its efficacy upon a more extended scale, and of ascertaining the efficient treatment of those cases in which magnesia is ineffectual, and in which it has even been found to aggravate the complaint.

To bring forward additional evidence in favour of the use of magnesia, and to distinguish the cases in which its use is indicated, from those where it is improper or hurtful, are the principal objects of the present communication, and will be considered in the two following sections.

For 1813, p. 106.

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The following is the tase of a gentleman who suffered from a calculous complaint, during which he was accidentally induced to employ magnesia, the effects of which he has thus described.

Case 1. About twenty-seven years ago, I felt a pain in one of my kidnies, particularly when in bed, which continued to increase during six months. I had likewise an occasional sympathetic pain in the testicles, and violent and excruciating pains in the left kidney now became frequent. These attacks were sometimes brought on by stooping to take up something ; but at other times without any apparent cause. They lasted from twelve to twenty-four hours, and I obtained some relief from the application of warm flannels; but they always left me languid and relaxed.

On the fourth attack I consulted a physician, who imagined that my complaint had been induced by drinking cyder, in which I had formerly indulged. He ordered me weak Hollands and water for common drink, and prescribed the lixivium of tartar to be taken in broth. This medicine was persevered in for some time; but I found it gradually weaken my stomach, and impair my digestive powers.

About nine months after my first attack in the kidney, I walked from Hampstead to London after dinner, and on the following day, I clearly felt something pass from the kidney to the bladder, and suspected what it was. I took about a pint of Hollands and water, and on attempting shortly afterwards to void my urine, found that the passage was blocked up, but had scarcely time to consider of my situation before the

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