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I have observed, in the Chapter on Choosing Telescopes," &c. that "a difference of 5 or 10 in the magnifying power will sometimes, on some objects— give quite a different Character to a Glass," and Mr. TULLEY states, in his Letter (see Chapter on Cassegranian Telescopes), that " on comparing a Gregorian and a Cassegranian Telescope, each of 7 inches aperture at a very little after sunset, he could then plainly perceive a difference between a power of 77 and 82-whichever Telescope had the power of 77, compared with the other at 82, was very visibly the lightest.”
A Good Achromatic of 22, or a Gregorian Telescope of 5 inches aperture (neither of which are near so light as the 7 feet Reflector of 6 inches aperture, which was the size of the Telescope Dr. H. here alludes to), when the planet is in a favourable position and near the meridian, will shew the Belt of Saturn, with a Double Eye-glass magnifying 100 times: and, on Dec. 16, 1824, at half-past nine, when the planet was close to the meridian, I saw the Belt in my 7 feet Newtonian which has a Metal of 6 inches, and was made by Sir Wm. Herschel, with Huygenian Eye-tubes, magnifying from 80 to 240 times. See Chapter on Saturn.
In a Single Eye-glass, especially if less than half an inch focus, whether Concave or Convex, the field of view is very small, and is distinct only in the very centre of it so that in a Telescope of 5 feet focus which magnifies more than 100 times-it must be kept in
almost constant motion to keep a Planet in the centre of it.
Saturn or Jupiter so very nearly fills all the distinct part of the field of a Single Eye-glass, that we cannot see the whole of them distinctly for more than two or three seconds without moving the Telescope : now, it is impossible to examine any object distinctly when it is in motion, and therefore, it is of great importance that the field of view be as large, and as uniformly distinct, as possible.
To my Eye, Saturn certainly appears sharper and brighter with a Single Concave or Convex Eyeglass-than I have seen it with any Compound Eyepiece, composed of two or more lenses: but I have not found this to be the opinion of persons unused to observe, which I suppose is because the Single Eyeglass is Distinct only just in the centre of the Field, and the difficulty of keeping the object there is so great to those who are unaccustomed to manage Telescopes, that I have found them invariably complain, that the Vision with the Single was less Distinct than with the Double Eye-glass.
See other Observations on the comparative light of various Eye-glasses, &c. in the last page of my Chapter on Diagonal Eye-pieces, and on Illuminating Power.
To conclude-the difference in brightness, &c., is so small, — the advantage of the increased and uniformly distinct field of view is so great, and the
Vision is so much more easy to the sight, that, especially for Telescopes of less than 7 feet focus, (excepting where very minute objects, such as faint double Stars, are to be examined, and the utmost distinctness of the central point is of more consequence than extent of field,) my Eye always asks me to use the usual Compound Eye-pieces-these require care in constructing of proper proportion, and careful exactness in putting together.
The odds in difficulty of construction against the Compound Eye-piece are, Two additional surfaces to grind and to polish, i. e. the imperfections of another Lens-and the centring of them perfectly true to each other-however, these difficulties are easily surmounted by a careful Optician, and thus the Huygenian Eye-tube is undoubtedly the most desirable.
"The aberration from the figure, where Two Eyeglasses are rightly proportioned, is but a fourth of what it must unavoidably be where the whole is performed by a Single Eye-glass." See Mr. DOLLOND's Letter to Mr. SHORT in the Phil. Trans. for 1753. And especially for Planets, and for Telescopes under 7 feet focus; because, when a single lens is shorter than half an inch focus, the aberration of Sphericity becomes extremely inconvenient for Planetary Observations.-Stars occupy so much smaller a portion of the Field of View, that for them it is of less importance.
THE invention of the Reflecting Telescope may be considered the epoch when Optical and Astronomical pursuits began to become general:-the unwieldy length of Refracting Telescopes, adapted to any important purpose, rendered them so extremely inconvenient, that it required the utmost dexterity to use them, as it is necessary to increase their length in no less a proportion than the duplicate of their magnifying power; so that, in order to magnify twice as much with the same light and distinctness, the Telescope required to be lengthened four times, and to magnify thrice as much, i. e. nine times the length.
This unwieldiness of the Refracting Telescopes possessing the needful degree of Magnifying power, caused the attention of Astronomers and Opticians, &c. to be directed to the construction of Reflectors; and, early in 1672, SIR ISAAC NEWTON completed his two small Reflecting Telescopes, which were but Six inches long, and were held in the hand for viewing objects, but in power were equal to a Six Feet
Refractor. See the Account of Sir I. N.'s Telescope in the Appendix.
For the following Tables of the proportions of GREGORIAN and NEWTONIAN Reflecting Telescopes, I am indebted to page 39 of the Appendix to the Nautical Almanack of 1787, which now being out of print and become scarce, I have copied here, from the same motives Dr. Maskelyne inserted them in his book.
TABLE of the Apertures, Powers, and Prices of Reflecting Telescopes, constructed in the Gregorian form, by the late ingenious Mr. JAMes Short*.
* Mr. Short charged higher Prices in 1768 than Opticians do now; although the price of labour and every article required in the