Those who have Telescopes which do not define Stars so well as they wish-may, supposing the aperture to be 2ths, make a pasteboard cover for the Object end, with an aperture of 26ths and if that be too large, contract it to 2ths, and so on by 10ths till the Image of a Star is neatly defined.

The best Advice I can give to Buyers of Telescopes, is, that if they are particular about the Quality of the Instrument, the less curious they are about the Price, the more likely they are to be pleased with the performance of it if they deal with an Optician of established character, and leave it to his Judgment and Integrity to choose for them-not restraining him in Price my own experience assures them, they have then, the best chance of obtaining what they desire.

For instance, a common portable Two feet Achromatic Telescope which is made merely for Day purposes and magnifies about 30 times, is sold for £4. 4s.;-if this is required to carry a power of 100, i. e. three times the power it was made to bear; and to define Double Stars, &c.—it must have a picked Objectglass, of a degree of perfection, which is

only attainable by a casual concurrence of the various circumstances which combine to form these Compound Object-glasses.

The Planet Jupiter was till within the last 30 years considered the grand test of Telescopes for Celestial purposes, — and when it is near to the Meridian it is a pretty severe one—but many Glasses will define a Printed Paper, and shew the Planets very well, which will not so well define Double Stars, because they were not adjusted at a Star. Double Stars were not thought of till the attention of Astronomers was called to them by Sir William Herschel publishing his Catalogues in the Phil. Trans. for 1782 and 1785-since that time, the Art of making Telescopes has been gradually improving, and both the Optical and Mechanical parts of them are now made much more perfect than they were Twenty years ago.

For exquisite perfection, -we are, in all mechanical matters, almost as much indebted to accident, as to Art: - for instance, a Watchmaker makes a Dozen Chronometers, and bestows an equal degree of attention to the finishing of each of them; so much so, that he has reason to hope they will all perform equally well : how

ever, when put to the trial, he commonly finds, that of the Dozen, -- perhaps Four, in spite of all his care and pains, will turn out but indifferent Watches,-Six of them good,— and the remaining Two, fine, and fit to

“Correct Old Time, and regulate the Sun."

But why Two of his Watches perform with such superior accuracy beyond the others he cannot imagine.

In every department of Art it is the same, the Acme of Perfection is always accidental, and the most experienced and pains-taking workman cannot attain it with undeviating certainty by any Rules this Observation applies to a Telescope even more than it does to a Watch-for the Optician has not only to contend with the difficulty of workmanship-but with the greater uncertainty of the quality of the Material he employs; and if not One in Half a Dozen Chronometers will measure Time truly—not one in a Dozen Telescopes which perform perfectly well for all other purposes, will define Double Stars sharply and accordingly, those which will, bear a proportionately High Price.

Very few persons, however, require Telescopes for this purpose. Objects which are so severe a test of a Telescope are as severe a trial to the Sight-and those who have due regard for their Eyes, forbear from straining them by all needless exertions.

See the account of an alarming Dimness of Sight, from such fatigue of the Eye, in pages 66 and 67 of this work.

CHAPTER XVIII.

OPERA GLASSES.

"Ne damnent, quæ non intelligunt."

THIS entertaining Optical Instrument, seems to have escaped the observation of preceding writers on Optics-the whole, of the "Lex Scripta" which I have seen about it, is, that "An Opera Glass" is like a Galilean Telescope, and composed of a concave Eye-glass and a plano-convex Object-glass-and that its Magnifying power will be augmented, in the proportion, that the focal length of the former is diminished, and that of the latter is increased.

As so little has been written on this subject—I have embraced this opportunity of endeavouring to communicate such information as I have collected concerning it. Few Persons know even

"How to Adjust an Opera Glass" partly from the want of this knowledge, and partly from the very low Magnifying power, and imperfect construction of Common Opera Glasses I have not been much surprised,

M