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Half as low-or Half as high

or Half as high as they ought such are only put to ordinary Telescopes, for the reason we have stated, that a low power is put to an ordinary Opera-glass.

But the Reader must not expect that every ordinary Telescope which is fitted up merely for Terrestrial purposes, will properly carry for Celestial purposes, extraordinary high Magnifying Powers, nothing like it; — nevertheless such Telescopes may be perfectly efficient Instruments for the purposes for which they are made, and perform .very well for Land Objects-there are plenty of Good Day Telescopes, but few Superlative Star Telescopes.

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To bear an uncommonly powerful Eyeglass, for Celestial purposes, you must have an uncommonly perfect Object-glass, and as the difficulty of forming this, increases as the magnifying power to be used with it increases in like manner, the makers must be rewarded for their trouble, — which is greater, as the Instrument must be adjusted at a Fixed Star, which is a much more elaborate and Eye-teazing operation than the ordinary method observed with Day Tele

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scopes, of the defining power of which, a Printed Paper is often considered a sufficient criterion.

If I was an Optician,-I think that I would almost as willingly — Waltz blindfold and barefoot among 9 Red hot Ploughshares laid at unequal distances from each other, as have All my Telescopes tried by that truly troublesome test a Fixed Star.

When a Telescope is perfectly adjusted,a very trifling accident-will derange it so as to prevent its properly defining a Star-although it may not perceptibly affect the brightness or distinctness of the vision of it with any other object—not even with the Planet Jupiter.

Before You condemn a Telescope because it does not very nicely define a Star, try it several Evenings with several Eye-pieces,—and let the Maker of it (trust it with no other Person) examine whether it be in perfect Adjustment.

Defects in Eye Glasses are seldom suspectedbut, however perfect the original power of the Object Glass or Speculum may be, it will avail little, if one of your Eye Glasses is veiny, &c., or not quite clean-or not exactly truly centred to the Object Glass. Sir Wm. Herschel's

Observation in p. 31, of the 95th Vol. of the Phil. Trans. is perfectly true :

"The best Eye-lens will give the least spurious diameter of a Star."

Not One Instrument in Twenty can be made to give a neat Image of a Star with its whole Aperture, and not Two of Them will give quite so perfectly well defined an Image with the Whole Aperture-as when it is to a certain degree contracted.

I do not think that any Achromatic of 2 inches Aperture, and 3 feet focus, can be made to give quite so neat an image of a Star with the whole of that Aperture, as a fine 5 feet of 3ths Aperture will when it is limited to 23I have never seen one that approximated within some degrees of it.

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The more perfect vision in the 5 feet, I am aware, is partly to be attributed to the greater original power of its longer Object Glass, and to its larger Eye Glasses-but what I have asserted is true of Telescopes of equal length; though the improvement is not in so high a ratio.

Whatever inexperienced Amateur Opticians may think of this declaration of " the Cook's Oracle," 'tis true.

The Author respectfully assures the Reader

are

that these Lucubrations from his Garret the result of actual experiments; and, like those which were lately published from his Kitchen-are faithful statements of facts repeatedly proved.

Although he has been very cautious, in constructing every sentence with words which would express his meaning as exactly and as clearly as He could, nevertheless, his chief Ambition has been to give those who may do him the honour of perusing this little Book, all the Information he has accumu lated, in the most convincing and most satisfactory manner; · and he has never been contented to offer a mere assertion, whenever it has been in his power to produce collateral proofs from the writings of experienced authors.

"If the Aperture of a Telescope be 5 or 6 inches, there will be required a piece of Metal 7 or 8 inches broad at least, because the figure will scarcely be true to the edges." See SIR ISAAC NEWTON's Letter to the Secretary of the Phil. Trans. March 26, 1672. vol. vii. p. 4032.

It may be supposed that Speculums are now worked with more accuracy than when "the Optician's Oracle," Sir I. Newton, wrote the above; however, I have not yet seen a

Reflecting Telescope of 7 Inches Aperture which did not define Stars much better when it was contracted in a certain degree.

That excellent Optician, and candid writer, Mr. Peter Dollond, observes of his Achromatics, that "though the surfaces of the Concave Lens, may be so proportioned as to aberrate exactly equal to the Conver Lens, near the axis, yet as the refractions of the two lenses are not equal, the equality of the aberrations cannot be carried to any great distance from the axis." See Mr. Dollond's Letter to Mr. Short in the Phil. Trans. for 1765.

I had an Achromatic of 3ths Aperture, which was a remarkably brilliant Day Telescope, and with which I saw Planets very well-but could not see Rigel and several other delicate objects distinctly, until its Object Glass was limited to 24 inches-with 3thsthe small Star accompanying Rigel was enveloped in the false light from the large one with 3ths inches-it was not much betterbut with 24 inches, the little Star was very distinctly, and easily visible, and it exhibited ε Boötis y Leonis, &c., better than I have generally seen them with Telescopes of 23 inches Aperture.

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