gree of Magnifying power! - this can only be accomplished by the Pancratic Eye-tube.

On the 25th of March, 1819, Mr. Pierce, the Optician, with a Pancratic Eye-tube which made a 1 foot Achromatic magnify 80 times, perceived a Geminorum to be double.

On the 5th of April, 1819, I.shewed this with a power of 70 to Mr. Wm. Brockedon, the Painter, and to Mr. Charles Turner, the Mezzotinto Engraver in Ordinary to His MA

JESTY.

a Geminorum requires very little Illuminating Power: I have shewn it to several persons who did not know that it was a Double star, with two 1 foot portable Telescopes, with an Objectglass of the usual aperture of 1 inch in diameter, to which I applied a Pancratic Eye-tube, which gave a power of 70 times, and they described to me its appearance very accurately.

I have seen these two Stars with 230 in an Achromatic Telescope of 28 inches focus, and 2 inches aperture, (which was made by the

* "THE EFFECTIVE POWER OF TELESCOPES has a considerable range of extent, — and can only be assigned when the Object to be viewed is given."- Sir WM. HERSCHEL, in Vol. CV. of the Phil. Trans. page 294.

present Mr. Dollond for the late Mr. G. Hodgson, at whose sale in February last I purchased it,) like Two Shillings on a bit of Black ClothSee diagram facing page 130; but in which, I beg to observe, that the smaller Star is made rather too small in proportion to the larger Star. See Sir Wm. Herschel's diagram of Castor, as it appeared in his 7 feet Newtonian with 460. in the Phil. Trans. for 1782.

ε Boötis, was observed on the 25th of May, 1819, by Mr. H. Browne, F.R.S. and myself, with an Achromatic Telescope of 27ths aperture, made by Mr. George Dollond, F.R.S. With a Pancratic Eye-tube magnifying 270 the two Stars, were just as perfectly and as distinctly defined, and at about the distance from each other, as represented in the Engraving of the Pancratic Eye-tube facing page 130, without either rings or rays, &c., around them. This was in an extremely fine clear evening - the air quite still and the star very near the meridian. The Blue colour of the smaller star was remarkably bright for so small an aperture -This Double Star is very rarely. seen perfectly distinctly, in an Achromatic with a less Aperture than 34, or in a Gregorian Re

flector of less than 5 inches, and a Power of 300.

Sir William Herschel saw it in his 7 feet Newtonian when its aperture was limited to 34 inches; with 460 the vacancy between the Stars was a diameter of the smallest. Phil. Trans. Vol. 95, p. 42.

See

Well might Dr. Maskelyne say that "Telescopes of Sir Isaac Newton's construction perform most excellently in the Minutia of Astronomy, especially if small Apertures and long Foci are made use of." See Supplement to the Nautical Almanac for 1787, p. 42.

Magnifying power when required for observing DOUBLE STARS, is also more perfectly effective in an Achromatic Telescope, in proportion, as it is derived from a proper degree of original power from the Object Glass- the image of the Stars appears smaller, and their separation greater.

I will here take the liberty to caution those who hereafter may be crazy with the Dumpy*

*This appellation was first given by Mr. SHORT, the celebrated Maker of Reflectors, to a Telescope he made for the Honourable Topham Beauclerc, of 6 inches aperture,

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

scopes are than long ones! I have lately 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