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Description of the Lamp.
(See Plate XV.) Fig. 1. describes the lamp as it appears when ready for use.
Fig. 2. A section of the lamp.
A. The body of the lamp constructed of copper or japanned iron, terminating in
B. A conical tube, which carries off the air (deprived of part of its oxygen by combustion) through the water in the cistern C.
D. Is a cistern containing water, in order to prevent the lamp from being over-heated.
E. The window of the lamp made of very thick glass.
G. A tube furnished with a cock, in order to bring the water to a level within the lamp.
H. A cistern containing water, which may be drawn off by the cock I.
K. A tube from the bellows which delivers air for the supply of the lamp, through the water in the cistern H.
Fig. 3. L. Spare bellows with an elastic tube M which may be adapted to a tube conveying pure atmospherical air, or to a gasometer. Forty gallons of atmospherical air will be sufficient to keep the lamp burning for one hour.
Fig. 1. N and O, two cocks to draw off the water from the cisterns C and D.
XXV. On the Light of the Cassegrainian Telescope, compared with
that of the Gregorian. By Captain Henry Kater, BrigadeMajor. Communicated by the Right Hon. Sir Joseph Banks, Bart. K. B. P.R.S.
Read May 27, 1818.
The Cassegrainian telescope from its first invention to the present time, has generally been considered to be merely the Gregorian disguised, and to possess no other advantages over it than the capability of being made shorter with the same magnifying power. This opinion, joined to the inconvenience of its inverting the object, has caused it to be thrown aside, perhaps too hastily, and without a sufficient examination of its properties.
As the experiments which I am about to detail may possibly lead to important conclusions, I shall perhaps be pardoned if I relate the circumstances which induced me to engage in them.
A self-taught artist of the name of CRICKMORE, who resides at Ipswich, had by exclusive attention to the subject, brought the Gregorian telescope to a degree of perfection surpassing any thing of the kind I have ever yet met with. Some months since, in the course of his experiments, he first completed a Cassegrainian telescope of one foot in length, and on viewing Jupiter with it, with a power of about 100, I was instantly struck with the brightness of the image, far exceeding what might have been expected from the aperture; but I supposed
this to have been a deception arising from the specula being more exquisitely figured than usual, which, producing greater distinctness, occasioned the idea of superior light. A short time after this, the same artist received an order for another telescope, which, from the success that had attended his recent efforts, he recommended to be of the Cassegrainian form. The aperture was five inches, the length thirty inches, and with a power of near 400 the image was so perfectly distinct and luminous, that I could no longer hesitate to conclude that, from some unknown cause, the Cassegrainian telescope actually possessed far more light than the Gregorian, and I waited most anxiously for an opportunity of verifying this, and determining the difference by experiment.
Such an opportunity soon presented itself, and under circumstances peculiarly favourable, as another excellent telescope of the Cassegrainian form was made, and I was fortunate enough to procure a Gregorian made by Mr. CRICKMORE some time before. The mirrors of both these telescopes were cast at the same time, and from the same pattern, so that no difference of light could arise from any difference in the composition of the metal. The magnifying power of both instruments was ascertained by experiment to be very nearly equal; but the excess was rather on the side of the Cassegrainian.
The telescopes being placed side by side, were directed to a printed card, at the distance of fifty yards; and on viewing it, the far superior brightness of the image in the Cassegrainian was strikingly apparent. Having prepared a circular piece of paste-board to close the end of the Cassegrainian telescope, I drew a number of concentric circles on it, at the distance of the twentieth of an inch from each other. The paste-board was then placed in the end of the tube, and an aperture was made, which was enlarged by cutting out one circle after another till the card appeared equally bright through both telescopes, and of this, the eye judges most accurately.
The following measures were then taken.
Gregorian Telescope. Diameter of the circular Diameter of the large Inch. opening in the paste- Inch. mirror
3, 90 board
2, 70 Diameter of the back of Diameter of the back of the small mirror 1, 09 the small mirror
1, 00 Length of the arm 0,805 Length of the arm
1, 45 Thickness
From the above measures the following calculations were made.
Inch. Area of the circular open- Area of the large mirror 11,946
ing in the paste-board 5,726 Area of the back of the Area of the back of the small mirror to be de
small mirror to be deducted
0,785 Area of the arm to be de. Area of the arm to be de. ducted
Area of the portion of the Area of the portion of the
mirror exposed to the mirror exposed to the
From this experiment it appears, that the light in both telescopes was equal when the area of the aperture of the Cassegrainian, was to that of the Gregorian, as 4,632 to 10,871. Now the increase of light being (under similar circumstances) directly as the area of the aperture, it follows that if the aperture of the Cassegrainian be made equal to that of the Gregorian, the light in favour of the former will be as 10,871 to 4,632, or in the surprising proportion of 7 to 3 nearly.
A difference of such magnitude could not be admitted but with extreme caution, particularly as the Gregorian telescope had been made some time, and its mirrors might therefore be supposed not to possess so high a polish, as those of the Cassegrainian which had been recently finished; but I was soon enabled to pursue the subject, as a Gregorian telescope was made by Mr. CRICKMORE fully equal, if not superior, to any he had before constructed; the inirrors were of an exquisite polish. The Cassegrainian, used in this experiment, was the one I formerly mentioned, the aperture of which was five inches, and the length thirty inches. It had not been carefully preserved, and the large mirror had lost somewhat of its original polish. All circumstances being thus in favour of the Gregorian, a paste-board circle was prepared, and the experiment conducted as before. When the images of the card were equally bright, the following measures were taken.