some hours during several Nights, in looking intently through Reflecting and Achromatic Telescopes, endeavouring to ascertain the comparative Illuminating powers of various Instruments, and the effect of variously constructed Magnifiers, for shewing the Division in the Ring of Saturn, and for separating some of the faintest and closest Double Stars-my Eye became so extremely tired that the sharpness of my Sight was so sensibly impaired that for two or three days after I hardly knew any face that was 20 feet from me, and became so much alarmed, that I mentioned it to an eminent Optician, who said, "Don't be uneasy, the same thing has happened more than once to myself; your Eye has been over-worked—give it a few days' Rest, and I dare say it will soon come round again." — His prediction speedily proved true. One of the tests to which I put my Eye, and my 5 feet Achromatic, which has a double object glass of 3 aperture, was, to ascertain with how Low, and how High a power I could see the small Star near the Pole Star: -it was a decidedly detached point of light with a single eye-glass, which gave the Telescope a

-

nifies

=

power of only 28, i. e. with the 2d E. G. of a compound Astronomical Eye-piece which mag44 times and it was visible with 20 intermediate Eye-tubes, the highest of which magnifies 1386 times and is a single Convex lens of the 22d of an inch focus. - The Object Glass being of

63 Inches focus,

gave, multiplied by 22

126

126

the Magnifying Power 1386.

This Instrument is one of the chef-d'œuvres of the late Mr. PETER DOLLOND- who thus speaks of it to the Gentleman he sold it to, Mr. G. Hodgson, F.R.S., (and of whom I pur chased it,) in his Letter, which is now before me, dated Nov. 11, 1803 : "It has been made 10 years, and I can say that it is one of the best I ever made, and such as I cannot expect to be able to equal."

This 5 feet Telescope literally deserves to be called Achromatic, for it shews the disk of the Moon and of Jupiter as white and as free from colour as a Reflector; to its perfect Achro

maticism I attribute in great measure its power of very distinctly shewing the division in the Ring of Saturn.

The visibility of the dark line, which proves the Ring to be double, depends not only upon the distinctness of the image formed by a Telescope, but on the power it has to shew a white object quite white, and a black one, black.

When Saturn is near the meridian, the division in its Ring is most plainly visible but it may generally be observed Two, and in very fine Nights, Three hours, before it comes to the Meridian.

Those who wish to see this in London, are advised that they will not discern the division in the Ring half so well before-as after 10 o'Clock.

In this "Elysium of Bricks and Mortar" we are so surrounded by "Groves of Chimneys," that until the majority, of the Great Fires of the great Manufactories are out - the obfuscated atmosphere of this monstrous metropolis defies all the penetrating power that the immortal Herschell himself could bring against it.

Objects which require all the powers of the Eye to be fresh, and in fine condition, should

not be examined when that Organ is tired with having been at work all Day.

If a Planet comes to the Meridian at Midnight, at 9 or 10 o'Clock lie down in a quiet darkened room, and rest your Eye by getting a nap previously. A little Horizontal Refreshment, you will find a proper and renovating preparative for such Contemplation; halfan-hour's Siesta will restore the tone of your Visual Organs, and sharpen your Sight prodigiously.

I also saw the small Star near the Polar Star with my Beauclerc Telescope, which has a triple Object glass of 46 inches focus, with a Lens of the 22d of an inch focus, which gave this Glass a power of 1012 times.

This singularly perfect Telescope, which shews Stars as stark neat as ever Nature permitted them to be seen;-I purchased at the sale of Mr. Aubert's Instruments, when they were sold at his Observatory, at Highbury Place, by Mr. S. Sotheby, July 24th, 1806:—it was originally fitted up for the Honourable Topham Beauclerc, and Mr. Ramsden's name is engraven on the Eye-end of the Telescope - but Mr. Peter

Dollond informed me that he made the Object Glass- and smiling at the time he gave me this information, said, "Yes, that Object Glass is one of the things which is to make me immortal," and I then gave him permission to engrave his title thereto on the tube of the Telescope.

To have produced such a perfect Instrument is so honourable to the talent of the artist, that, to avoid all appearance of prejudice to either of these eminent men, I have called it by the name of the person it was made for, Beauclerc.

In the Second Part of "THE ECONOMY OF THE EYES," which is now preparing for the Press,

"Paulo majora canamus”

the Illuminating and Magnifying Powers of NEWTONIAN, GREGORIAN,CASSEGRAINIAN,- HERSCHELIAN Reflectors, and GALILEAN, HUGHENIAN and ACHROMATIC Refracting TELESCOPES of various sizes,—will be explained by Experiments made with 46 Telescopes of from 3 Inches to 7 Feet focus, which I purchased for that purpose, and have cost me upwards of £1200; and which all who