Mania, that the convenience derived from Telescopes being made short, (if beyond a certain proportion,) is greatly more than overbalanced, by the errors produced by the great increase of the aberration of Sphericity arising from the deep curves of the excessively small Eyeglasses we are obliged to employ - there is much difficulty in getting deep Lenses well worked and so much more mischievous are

the errors arising from any deviation from proper figure.

It is almost impossible to find an Eye-glass so deep as even the th of an Inch focus, that will give a well-defined image of a Star, notwithstanding much deeper magnifiers are useable in Microscopes.

Steady Stands are now constructed at a very moderate expense, which make it as easy and which I saw in Colonel Aubert's Observatory at Highbury; it was only 24, instead of the length he usually made them, i. e. 36 inches focus. The instrument is well known in the Optical World by the name of " Short's Dumpy."

Mr. Tulley informs me, that this Telescope is still in high preservation, and is now in the possession of Mr. Allen, Plough Court, Lombard Street.

* See the Supplement to the Nautical Almanac for 1787, p. 39.

to use a Telescope of 9 feet, as one of 3 feet

in length.

Writing the above, reminds me of a conversation I had about 25 Years ago with an eminent Optician of great experience:

Kit. How much more convenient short Tele-
I have lately

scopes are than long ones!
bought a Dumpy.

Opt. Do you find it perform much better, Sir, than Telescopes which are of the usual length?

Kit. No, it certainly does not perform better. Opt. Did you pay much less Money for it then? Kit. No, Sir a great deal more.

Opt. Then I think, Sir, that You have laid out your Money very badly-I guess that You have not got so good a Telescope for £30. as You might have had with half the trouble to yourself and the Optician for £20.- for You might have had the choice of half a dozen Telescopes of the usual length, and what does it signify whether the Tube is 2 or 4 feet long?-a Stand that will carry the one will carry the other; and remember, Sir, that Vision is better, and easier to the Eyein proportion that Magnifying power is pro

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.

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.

ments

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 appears - for want 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 still

and 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