have Instruments of the like dimensions, may easily verify at the expense of a few shillings. A circumstantial account will be given of their several performances and powers, as Astronomical and Day Telescopes, - being the results of 30 years' observation. Concave To enable my Readers to measure the respective merits, and the relative reflective powers of Convex, and Plane Small Specula-I shall lay before them, an abstract of their several pretensions as stated by preceding writers thereon; then proceed with those Practical Observations which I have made myself, and those which I have been favoured with by several scientific Opticians, and amongst them those experienced and excellent Makers of Reflecting Telescopes, Messrs. Watson Tulley -- and Cuthbertwho have given me their Notes and Observations thereon, and the account of the Facts which they have actually ascertained in the course of their numerous experiments with Telescopes of various constructions. I have only room in this page to add, that the Opinions of these eminent Practical Opticians are perfectly unanimous, and in perfect unison with those published by Sir I. Newton, and that I believe, that their Evidence and Arguments are so true, and so convincing, and so plainly stated, that they will be perfectly satisfactory to the Reader, and will finally settle certain important points, which without such Illustration, seem to me, likely to remain as they are at present— "Puzzled with Mazes, and perplexed with Errors." I shall also give an account of the invention and advantages of the PANCRATIC EYE-TUBE, which to a 3 Achromatic, gives in the most perfect manner every degree of Magnifying power from 100 to 400 times-which will be minutely described and explained by Engravings. N. B. Contributions on Optical Subjects, ad dressed to the Publishers of this Work, will be gratefully acknowledged. I have mentioned my own case of dimness of Sight, because, I believe many Artists, from the want of such a hint, have greatly injured their Eyes, by having been induced by similar Symptoms, prematurely to put on Spectacles: but, in such cases," Rest is the best Remedy." "So great is the calamity entailed upon the makers of fine Pieces of Workmanship, such as Clocks and Watches, that many of the Workmen are almost Blind before they arrive at Old Age. I know a Jewish Woman in this City that had a peculiar way of stringing of Pearls, so as to cover their blemishes if there were any, and by that means got a deal of money; but when she came to be forty years of age, finding no manner of relief from Spectacles, she was forced to leave off the Business. I remember likewise to have heard several Printers complain, that they have given a considerable shock to their Eye-sight by composing small types. "In earnest, I do not see how we can afford any Relief to the Workmen we now treat of; for it is not easy to persuade them to leave a beneficial and lucrative trade; and Physic is unprovided with any Remedy that can restore the primitive Strength and Mobility of the Eyes, after the Disorder has become inveterate: for neither Purging, nor Bleeding, nor other Medicinal Means, can take place in this Case, in regard the Patients are otherwise Well and Brisk, and their Spirits being neither clouded nor incrassated, it would be improper to punish an innocent and sound Head with the Commotions of Medicines. "However, I would advise such Workmen not only to use Spectacles, but to intermit from their Work now and then, and refresh their Eyes by Diversity of Objects. For we can't imagine How much the Mobility of the Membranes of the Eyes, and the native Fluidity of the Humours, is kept up by viewing divers Objects; some near at hand, some remote, some directly, others obliquely; and, in fine, all manner of ways: for by this Means the natural Disposition of the Eye is preserved, so that the Ball is sometimes contracted, and sometimes dilated; and the Crystalline Humour approaches more or less to the Pupilla, according as the remoteness or nearness of the Object requires. Without this Diversity of Action, the Eyes undergo the same Fate with the other parts, that, by being long detained in one position, grow stiff and unfit for Motion."-RAMMAZZINI on the Diseases of Tradesmen, chap. xxix. of the Eng. Trans. 8vo. 1705, p. 219. WATCH-MAKERS, ENGRAVERS, and those who are in the habit of using Strong Magnifiers, would feel their Eyes much less fatigued, if the objects they examine were always placed at once, and kept at the proper focus : this might be contrived very easily, by fixing the Magnifying-glass in the opening of a Spectacle frame, or on a stand, and making a mark where the object of examination is most distinct. Nothing can be more detrimental to the organ of sight than the clumsy practice of holding a glass by squeezing the orbicularis muscle, this cannot be done without distorting, and distressing, and much injuring the mechanism of the Eye. it, The less the Magnifying Power of the Glass, the less the Eye will be fatigued by - the less distressing the position of the Body in working with it, and the larger and more uniformly distinct the field of view: and where a moderate Magnifying power is sufficient instead of a single magnifier, I think it will be better, especially for Etching, and for examining the general Effect of Engraving, &c. to wear Spectacles of 9 inches focus with which I think that Artists might work longer than with only one Eye. -- The Compound Magnifiers, which are composed of two plano-convexes with their plane |