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to be decidedly more brilliant than Gregory's, because more light is reflected by the Plane* small speculum of the Newtonian, than by the Concave small speculum of the Gregorian;-and in the Newtonian, all the Magnifying powers being produced by changing the Eye-pieces, may easily be made equally good, and admit of the application of almost an endless variety of Eye-glasses.
Another great advantage of the Newtonian Telescope is that its Eye-pieces are unencumbered with Eye-holes and all kinds of single and compound Eye-tubes may be applied thereto, and it affords an opportunity of proving the peculiar properties and powers of every kind of lens.
The Makers of GREGORIAN or CASSEGRANIAN Telescopes, say that those instruments cannot be made equally perfect with the extremely Low and extremely HIGH powers, if the change of magnifying is produced by changing the small Specula-which are seldom equally good.
I have heard the superiority of SHORT'S REFLECTORS attributed to the patient industry with which he worked his Large metals, and the very great care he bestowed in adapting the Small spe
* This, I am told, is a most difficult thing to obtain; and most of those that pass for PLANES, are in fact, either Concaves or Converes of thirty or forty feet focus. -See note at the end of the 5th Chapter on Diagonal Eye-tubes.
culum to the large one; he made a great many small specula of the same focus, and tried them one after the other, till he made a good match.
It would much improve these instruments, as well as render them more convenient for use, if Eye-pieces were employed as in the Newtonian, (see an Account of the Three or Four Feet Gregorian in the following Chapter): still the Newtonian would be superior for astronomical purposes, from the greater quantity of light it reflects, and from the pleasant position in which we observe, especially for viewing objects in high altitudes, when, instead of almost dislocating one's cervical vertebræ-we look comfortably straight forwards.
The Newtonian stand, as now made, with Four Feet, on Castors, (which ingenious piece of mechanism was contrived by Dr. Herschel,) still admits of improvement, by being placed on Three feet, two behind on Castors, and one before without a Castor. I have thus altered the stand of my Herschel Newtonian, and it is much more steady than when it had Four feet, one of which was always Dancing.See in Chapter VIII. on 7 feet Newtonians, an account of Mr. Newman's Improved Stand.
THE smallest Gregorian Reflector that is usually made, is
THE 1 FOOT;
this has an Aperture of 24 inches, and has generally one Eye-tube, and Two small Speculums magnifying about 50 and 90 times.-Two Eye-tubes would be much better than Two small speculums-because they are easier changed, and the lower power may then serve for a Finder to the high power; I advise this, especially if the Instrument is to be used for Astronomy: for I have always found it impossible to change the Small Specula without a light to shew the Eye how to direct the arm to the groove, which carries the small metal, and the light puts the visual organ out of tune for several minutes.
The Vision of this Telescope is superior to the 2 feet Achromatic, which has an aperture of 16 inches, but is not quite equal to the 30 Inch of 2 inches aperture.
THE 18 INCH,
with an aperture of 3 inches, is superior to the Re
fractor of 30 inches focus, but is seldom made; the former size Reflector being preferred by those who want a Portable Telescope for Day purposes, and those who wish a more effective fixed Instrument choose
THE 2 FEET,
which has an aperture of 4 inches, and is as powerful a Telescope as our atmosphere, near the Horizon, will generally permit us to use for Terrestrial purposes, and for use at a Window, is the Instrument to be generally recommended-its shortness makes it much more convenient than an Achromatic, which, of equal power, must have an aperture of 22 inches, and be of 44 inches focus, as I have stated in my Table of the Comparative Illuminating Powers of Achromatics and Gregorians."-See Chapter XVII.
I have a Gregorian Telescope of 4 inches aperture, and 15 inches long, made by Mr. Watson, which shews the Belts of Jupiter and the Division in Saturn's Ring, as well as I have seen these objects in my 3 Achromatic of 2 aperture.
This Gregorian shews Double Stars (see OBS. on Castor, in the 20th Chapter,) with a sharply defined disk, like little Planets.
The Gregorian of 4 inches aperture is often made of 19 in length, which is very handy, and of this length it may be made as perfect as of 24 inchesbut as the Speculum requires more care in figuring, for such shorter instruments the Makers charge more money.
This Telescope has usually Two Huygenian Eyetubes and Two small Speculums, magnifying 60, 90, 120, 180 times. I would recommend the addition of an Eye-tube of 40 for the Moon, and another to magnify 130 times with the lowest small Speculum-then there will be no occasion to change the small metals for terrestrial purposes, or for observing Planets only for examining Double Stars, which require the higher magnifying powers, and for which purpose it will very well bear a power of 250, which the Eyepiece that gives the power of 130 to the largest Small Speculum will give to the Smaller one.
Each of the Eye-tubes of a Gregorian Telescope may be made to serve the purpose of producing Three magnifying powers-by merely having additional Eye-heads-and using the First Eye-glass alone and the Second Eye-glass alone-which will give powers of similar proportions to the compound Eye-tube as those mentioned in the account of the 5 feet Achromatic, in Chapter IV.-and see also the word Huygenian Eye-piece in the INDEX.
This Telescope must be furnished with a Finder, which should be a 1 foot Achromatic, with an erect Eye-tube magnifying 10 times as it will be useful for Terrestrial as well as Celestial purposes-as the vision is erect in the Telescope, it ought to be erect in the Finder also.
This is the largest Gregorian that can be used pleasantly on a simple Pillar and claw stand, on which it must be nicely balanced when the Cover of