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"The change from one Power to another may be made instantaneously, with the utmost facility and certainty, and the Observer always knows exactly what Power he is using."
[The above is an extract from Dr. Kitchiner's Letter to Sir JOSEPH BANKS, P.R.S., which was read at the meeting of the R.S. on the 20th of April, 1820.]
Another is made, which is adapted for TERRESTRIAL purposes, Magnifying with an Achromatic of 44 inches focus, from 55 to 200 times and with a 30 inch from 40 to 160.
The power it will give to a Telescope, is according to the focal length thereof, and is easily found by the Rule of Three; for instanceif the Telescope be of 18 inches focus
If a Telescope of? what will one of 18
44 Inches gives
from 22 times, up to 88-being as low a power as is requisite for Land Objects-and as high as is requisite to shew the Ring of Saturn, the Belts and Satellites of Jupiter and several Double Stars.
If the Pancratic be applied to a Telescope with sliding tubes, it is desirable, on account of the great power it produces, that it should have an Adjusting Screw with a tooth and pinion; these are made separate from the Telescope, and introduced between the first and second sliding tubes.
The Advantage-of having ALL Powers in ONE Eye-tube is sufficiently obvious.
In very Clear Days the Air is so transparent, that we can use a Power of 100 for Land Objects, and on objects well illuminated sometimes 150, as well as in some other days we can a power of 50. Telescopes act best when used in the same direction that the Sun shines.— I have known good Telescopes condemned by trying them upon objects situated towards the East in the morning, or the West in the evening.
To have Perfect Vision every Day, and every Object, must have its appropriate de
gree of Magnifying* power! - this can only be accomplished by the Pancratic Eye-tube.
On the 25th of March, 1819, Mr. Pierce, the Optician, with a Pancratic Eye-tube which made a 1 foot Achromatic magnify 80 times, perceived a Geminorum to be double.
On the 5th of April, 1819, I.shewed this with a power of 70 to Mr. Wm. Brockedon, the Painter, and to Mr. Charles Turner, the Mezzotinto Engraver in Ordinary to His MA
a Geminorum requires very little Illuminating Power: I have shewn it to several persons who did not know that it was a Double star, with two 1 foot portable Telescopes, with an Objectglass of the usual aperture of 1 inch in diameter, to which I applied a Pancratic Eye-tube, which gave a power of 70 times, and they described to me its appearance very accurately.
I have seen these two Stars with 230 in an Achromatic Telescope of 28 inches focus, and 2 inches aperture, (which was made by the
"THE EFFECTIVE POWER OF TELESCOPES has a considerable range of extent, and can only be assignedwhen the Object to be viewed is given."- Sir WM. Her
SCHEL, in Vol. CV. of the Phil. Trans. page 294.
present Mr. Dollond for the late Mr. G. Hodgson, at whose sale in February last I purchased it,) like Two Shillings on a bit of Black ClothSee diagram facing page 130; but in which, I beg to observe, that the smaller Star is made rather too small in proportion to the larger Star. See Sir Wm. Herschel's diagram of Castor, as it appeared in his 7 feet Newtonian with 460. in the Phil. Trans. for 1782.
ε Boötis, was observed on the 25th of May, 1819, by Mr. H. Browne, F.R.S. and myself, with an Achromatic Telescope of 27ths aperture, made by Mr. George Dollond, F.R.S. With a Pancratic Eye-tube magnifying 270 the two Stars, were just as perfectly and as distinctly defined, and at about the distance from each other, as represented in the Engraving of the Pancratic Eye-tube facing page 130, without either rings or rays, &c., around them. This was in an extremely fine clear evening — the air quite still and the star very near the meridian. The Blue colour of the smaller star was remarkably bright for so small an aperture This Double Star is very rarely seen perfectly distinctly, in an Achromatic with a less Aperture than 34, or in a Gregorian Re
flector of less than 5 inches, and a Power of 300.
Sir William Herschel saw it in his 7 feet Newtonian when its aperture was limited to 34 inches; with 460 the vacancy between the Stars was a diameter of the smallest. See Phil. Trans. Vol. 95, p. 42.
Well might Dr. Maskelyne say that "Telescopes of Sir Isaac Newton's construction perform most excellently in the Minutia of Astronomy, especially if small Apertures and long Foci are made use of." See Supplement to the Nautical Almanac for 1787, p. 42.
Magnifying power when required for observing DOUBLE STARS, is also more perfectly effective in an Achromatic Telescope, in proportion, as it is derived from a proper degree of original power from the Object Glass-the image of the Stars appears smaller, and their separation greater.
I will here take the liberty to caution those who hereafter may be crazy with the Dumpy*
*This appellation was first given by Mr. SHORT, the celebrated Maker of Reflectors, to a Telescope he made for the Honourable Topham Beauclerc, of 6 inches aperture,