The stops is placed behind the convex speculum, in order to prevent the direct rays from the object, which would pass unrcflected through the openings in both specula, and fall upon the eye-glass, from mixing with the rays regularly refIccted'by the specula, and forming the magnified image upon the retina. See Smith's Op tics, Vol. II. Remarks, p. 94.
The next microscopes which excited any notice, were those of M. Delebarre of the Hague, who began to con struct them in 1771. Montucla saw them when he tra velled in Holland in 1773, and induced Delcbarre to go to Paris, where he was well received, and where he sold many of his instruments, each of which cost about 1 V.* M. M. Montigny, Lcroi, and Brisson, submitted to the Academy, on the 21st of June, 1777, a most flattering report of their performance, and Montucla has given a very full account of them in his History of Mathema tics. We have read these eulogiums and descriptions with great attention, and are obliged to acknowledge, that we cannot find in the microscope of Delebarre any improvement of the least importance which had not been known and adopted by the London opticians. The only point of difference that we can observe is, that of changing the eye-glasses, and of combining six lenses at once, which Montucia seems to lay considerable stress upon, because Euler, in his paper De ../Vuvo crosconiorunz genere, published in the memoirs of St. Petersburgh for 1766 and 1767, had pointed out certain theoretical advantages which he conceived would be found in microscopes with six lenses. Such a combina tion of lenses, however, has been exploded, and we do not scruple to say, that there is no eminent optician in Europe who would construct a microscope in such a manner, unless the lenses were arranged, as in some mc dern eye-pieces, so as to correct the chromatic aber ration.
M. £pinus of St. Petersburg]) proposed in the year 1784, ?ct. Petrol!. 1784,) to construct microscopes with large apertures of considerable length, with .chro matic object-glasses. The great object of this construc tion was to permit light to be easily thrown upon the object ; and in order to put his idea to the test of ex periment, he constructed a microscope, in which the aperture of the object lens was about 1 inch ; the dis tance of the object from the object-glass 7 inches, and the length of the whole instrument a little less than three feet. The magnifying power was from 60 to 70,
and therefore the focal length of the eye-glass must have been about an inch, so as to magnify by itself 12 or 13 times. This instrument is said to have given great satisfaction. The same idea had occurred long before to our countryman Benjamin Martin, who, in his description and use of a polydynamic microscope, has shewn that the achromatic perspective may be easily applied for this purpose.
Various improvements were made upon the micro scope by Mr. Cuff, Mr. Benjamin Martin, Mr. Adams, and other opticians ; but as they relate principally to the method of fitting them up, and of rendering them more commodious and universal in their applica tion, we cannot enter into any detailed account of their respective improvements, which will be better seen in the account of the instruments themselves.
Having thus given a short account of the history of the compound microscope, we shall now describe some of the most interesting Forms in which it has been fit ted up.
I. Curs Double Constructed Microscope.
This instrument is represented in Plate CCCLXXVII. Fig. 19, where ABC is the body of the microscope, hav ing an eye-glass at A, an amplifying lens at B, and a magnifier screwed on at C, and shewn separately at Q. The body of the microscope is supported by the arm DE, fixed on the sliding bar F. The principal pillar a b is fixed on the box b e, and the brass foot d is screwed to the mahogany pedestal XY, having a drawer to con tain all the apparatus. The bar F is tightened by a milled headed screw 0, when the adjusting screw c g is used. The objects arc laid upon the stage p g, with a hole n in its centre, and the light thrown upon them by the concave mirror G.
The following are the different parts of the apparatus vvhich accompany the microscope: 11 a convex lens for concentrating the rays of the sun upon the objects.
L a cylindrical tube open at each side, and having a concave silver speculum screwed to the lower end h.