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duced by Eye-glasses of long foci.—I am taking it for granted, Sir, that the Instrument is employed for important scientific purposes, when the first consideration is Optical perfection. However, I ask your pardon, Sir, for speaking so plainly—perhaps You purchased your Dumpy merely for a Plaything?

I have seen & Boötis, as distinctly as represented in the diagram, facing page 130, with a Telescope which would not exhibit a glimpse of the small Star which accompanies Rigel, nor the small star near the Pole Star and other Telescopes which would plainly shew the latter, but failed entirely at the former.

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Very few Instruments are so perfect, that they will perform perfectly well on all Objects. -There is almost always, some false light flitting about some part of the Image, and if a small Star happens to be in that part, it is enveloped therein, and is

"Invisible or dimly seen."

I do not call it seeing a Star Double, when you can only now and then, fancy you can perceive a faint glimpse of a little flitting ghost of an accompanying Star, during fits of easy

transmission-but only, when the apparent diameters of the Two Stars are as perfectly round, well defined, and distinctly separate, with a deep black division between them: as they are delineated in the diagrams in the plate facing page 130.

I have several times seen that very pretty object, y Andromeda, with 1-foot Achromatics, with an aperture of 1 andth of an inch, and a Magnifying power of 35. In these little telescopes, the smaller Star which in larger instruof a fine Blue colour for want

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ments appears of illuminating power, appeared of the same colour as the larger Star. The Blue Colour of the Stars accompanying this Star and & Boötis, becomes vivid in proportion to their proximity to the Meridian, and the Perfection and Illuminating power of our Telescopes.

I must here caution the Novice, that He must not often expect to see these extremely minute objects to the utmost advantage*, as I

"For if there be any vapours moving and undulating in the atmosphere, which often happens, though the night appears clear to the naked eye, these will entirely destroy the distinctness of the appearance: and it often happens that the air in this respect, at least here with us at Kew,

have described them, when I saw them at very favourable moments with very fine Instruments especially the Colour of the Blue Stars, even when they are near to the Meridian, and the Illuminating power of the Telescope is in due proportion to the Magnifying power, and the Instrument is Extremely perfect unless the Air is very clear and stilland every circumstance is favourable *.

will so suddenly and so totally alter, that the object will appear very distinct and very confused afterwards in 3 or 4 seconds of time; and the air is sometimes so very variable that objects will appear instantaneously to change, from being very clear to be confused, and then to be clear again. It will therefore be proper to accustom one's self to the fluctuating appearances of some land-objects, seen in the day time through the reflector; lest the undulating appearances of the planets in the night may deceive one, and incline one to think this instrument does not succeed so well as it is certain it will in a pure undisturbed Air."Dr. SMITH'S Optics, 4to. vol. ii. p. 366.

*"I have had recourse to my Journals to find how many Favourable Hours we may Annually hope for in this Climate.

"It is to be noticed, that the nights must be very clear- the Moon absent -no Twilight-no Hazinessno violent Wind- and no sudden change of Temperature; -and it appears that a Year which will afford 90, or

The Astronomical Amateur should be fully aware, that such is the capricious and vibrating state of the Atmosphere of this Country, that many Evenings which seem to be extremely fine, when the Stars appear very brilliant and dazzling to the naked Eye, are quite unfit for Observation, and our best Telescopes will perform but very badly.

Sir Wm. Herschel observes, that "Double Stars require a great deal of good distinct light; or even with the best instruments, the observer must not condemn either his instrument or his eye if he does not discern them."

N. B. The apparent Diameters

and the Distances of Double Stars* from each other,

at most 100 Hours, is to be called a very productive one." ! ! ! — Sir WM. HERSCHEL, in page 84 of the 90th vol. of the Phil. Trans.

* The Reader will find, in the 26th Number of The Journal of Science, edited at the Royal Institution, 1822, 24 Diagrams of Double Stars, by Mr. J. SOUTH, F. R.S. In the Second Part of the ECONOMY OF THE EYES will be given Portraits of the Planets, and Diagrams of the most remarkable Double Stars, as they appear in Telescopes of various Magnitudes, with various Magnifying powers stating with how Low a Power and how Small a

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vary very much, according to the different states of the Atmosphere, -the Defining,Illuminating, and the Magnifying power of the Telescope, and their proximity to the Meridian.

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To avoid the painful position when observing Celestial objects when they are near the Zenith, I have tried all the 9 different kinds of Diagonal Eye-tubes, the respective advantages of which I shall state at large in the Second Part of this work; I have only space here to say, that I think that the best I have was made by Mr. G. Dollond, and consists of a Plane Speculum placed at an angle of 45 degrees between the Object Glass and the Eye-glasses, and receives all the Eye-pieces, and renders an Achromatic Refracting Telescope as convenient as a Newtonian Reflector.

The light lost in the Reflexion is scarcely perceptible when observing fixed stars-the position is not only pleasanter, but our organ of sight is more perfect, when we look comfortably straight forwards-than it is in the break-neck position required in observing ob

Telescope, the Stars may be perceived to be separate, and with What Power they are seen best.

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