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or Opera Glasses which have a Concave Eyeglass-The Field of View, when they do not magnify more than Twice, depends in a great measure on the Diameter of the Object-Glass.

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THE Editors of former Optical and Astronomical works seem not to have been sufficiently sensible of the importance of presenting to the Eye accurate Portraits of the subjects of the Astronomer's contemplation.

Nature has given Eyes to all, an Understanding to few. Ocular demonstration is not only more desirable per se, but is more generally comprehensible than description; in the ratio, that more men can see than can reason.

However, it does not appear to me that any of the representations of the Moon or Planets given in former publications have been taken from Nature(excepting Russell's moon, and Dr. Herschel's prints, in the Phil. Trans.): the Moons are miserable imitations of Hevelius's or Cassini's; and the only drawings of Saturn that in anywise resemble the

Planet, are bad copies of that made with Mr. Hadley's Newtonian Reflector in 1723; or those given by Huygens in his Systema Saturnium, 4to. 1569, in page 16 of which work he has given a drawing of the Planet without its Ring, as it appeared on Jan. 16, 1656; in p. 18, the reappearance of the Ring and in pp. 21 and 24, two figures, which latter are such good representations of Saturn, I am surprised to find neither Belts nor Division in the Ring depicted, which I think must have been visible to him, if he saw the Planet as large as his print, and as well defined; in p. 35 are the 13 monstrous strange pictures of Saturn —which are copied in Dr. Smith's Optics, &c.; and in p. 55 is the drawing of the phases of the Ring, which has been copied into most of the elementary works on Astronomy.

Most of the other Portraits of the Planet, &c. are about as much like the Originals, as the sign of "The Seven Stars" on the alehouse at Brentford Butts is like the Pleiades.

The pictures of the Planets heretofore given in Astronomical works, seem to have been painted by the Imagination, rather than with the Eye, and remind one of the productions of some of the primitive Painters, who, with a modest consciousness of their lack of ability in their Art, or the want of discrimination in the Spectators, wrote under their Pictures, "This is done for a Black Lion;" or, "This is

a White Cat;" and instead of the Picture illustrating the press, the latter was employed to explain the former.

I consider it the chief merit of a Print that it is an accurate representation of the actual appearance of the Planet as seen in a powerful Telescope.

The engraving of Saturn, represents that Planet as it appeared in 1824, through my Herschel 7 feet Newtonian, with 6ths inches aperture, and an Huygenian Eye-piece magnifying 213 times and in a 5 feet Achromatic, of 3ths aperture, magnifying 190 times; and I trust will be acceptable to those who have not an Instrument of sufficient perfection and dimension to shew the Original.

It is sometimes no easy matter to make a Novice see either the Belts of Jupiter-or the Belt on the Body and the Division in the Ring of Saturn, the separation of Double Stars, &c., it is difficult to imagine what appearances are described by those Words-but when they have been pointed out in a Portrait of them, I have found people discern them directly-and candidly declare, that they knew not before what they were to look for. It is much easier to see an object when it is pointed out to us, than when it falls in our way unexpectedly, especially when of such a nature as to require some attention to be seen at all."-Dr. HERSCHEL, in Phil. Trans. for 1782.


Therefore, those who wish to entertain their Friends with exhibiting to them these Celestial Phenomena - should first clearly explain to them, what they are to see, by shewing them a Portrait of the Object,―then, give them a general view of it with a low power of 60 or 80, which having a large field will allow the Planet to remain in view for a couple of minutes and then refer again to the Portrait of it-when thus prepared, the Exhibitor may proceed to apply such Magnifying powers as are best suited to the size of the Telescope and the nature of the Object to be observed.

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A mode of calculating the due degree of Magnifying power for each Telescope and each object, I shall, in the following pages, I hope, succeed in explaining in the clearest manner.

A Telescope for shewing other Persons, should have for a Finder a one foot Achromatic with a power of 15 times, with several cross wires; this should stand out so far from the Telescope that One Person may conveniently look through it, while another is observing through the Telescope; and by means of Rackwork the person looking through the Finder may easily keep the object in the field of the Telescope, provided it does not magnify more than 130 times; almost as convenient a mode of observing with a common Finder may be produced by fixing before the Eye-tube a plain small

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