moveable tube is drawn out. It has also a second and third set of lenses, corresponding to C and D of a shorter focal distance, which produce higher magnifying powers on a principle to be afterwards explained. _Description of an eye-piece, &c. of an old Dutch Achromatic Telescope._ About twenty or thirty years ago, I purchased, in an optician’s shop in Edinburgh, a small achromatic telescope, made in Amsterdam, which was supposed, by the optician, to have been constructed prior to the invention of achromatic telescopes by Mr. Dollond. It is mounted wholly of brass, and in all its parts is a piece of beautiful and exquisite workmanship, and the utmost care seems to have been taken to have all the glasses and diaphragms accurately adjusted. The object glass is a double achromatic, 6-1/2 inches focal distance and 1 inch diameter, but the clear aperture is only 7/8 inch diameter. It is perfectly achromatic, and would bear a power of 50 times, if it had a sufficient quantity of light. The following inscription is engraved on the tube adjacent to the object glass:--“_Jan van Deyl en Zoon Invenit et Fecit, Amsterdam, Ao. 1769_.” Although Dollond exhibited the principle of an achromatic telescope, eight or ten years before the date here specified, yet it is not improbable that the artist whose name is here stated, may not have heard of Dollond’s invention; and that he was really, as he assumes, one of the inventors of the achromatic telescope. For, the invention of this telescope by Dollond was not very generally known, except among philosophers and the London opticians, till a number of years after the date above stated. Euler, in his “Letters to a German Princess”--in which telescopes are particularly described, makes no mention of, nor the least allusion to the invention of Dollond, though this was a subject which particularly engaged his attention. Now, these letters were written in 1762, but were not published till 1770. When alluding to the defects in telescopes arising from the different refrangibility of the rays of light, in Letter 43, and that they might possibly be rectified by means of different transparent substances, he says, ‘But neither theory nor practice have hitherto been carried to the degree of perfection necessary to the execution of a structure which should remedy these defects.’ Mr. B. Martin, in his ‘Gentleman and Lady’s Philosophy,’ published in 1781, alludes to the achromatic telescope, but speaks of it as it were but very little, if at all superior to the common refracting telescope. And therefore, I think it highly probable that Jan van Deyl, was really an inventor of an achromatic telescope, before he had any notice of what Dollond and others had done in this way some short time before.