Euphrof. I think this appears to be the most convenient Method of them all, as I have nothing to do but only fit at the Table and look through the Glafs at the Objects.But I fee, that in this Sort of Microscope you have different Conftructions; for I take it, that all these before me are Specimens of each particular Form. Cleon. They are fo; and I have procured them, in order to give you an Idea of the different Manner of applying, and viewing Objects in this Way. In all of them there is a Variety in the Frame in which the Glaffes are placed, the Method of conftructing the moveable Stage by adjusting Screws, &c. the Manner of fixing the reflecting Speculum, and many other Peculiarities, which you will eafily become acquainted with in the separate Use of each. Some of thefe are very expenfive in regard of curious Workmanship :-Others, too much laboured and incumbered with too many Parts and Appendages:-And others are as remarkabie for their light, eafy, and elegant Conftruction one of this Sort I shall here recommend to you for your common Ufe, and it confifts of the following Parts: (1.) A Piece of Brafs properly formed for the Foot or Bafis, with a circular Hole in the Middle, whose Ufe I fhall mention hereafter. (2.) The Stem, which confifts of two Parts; The 1ft of which is a Pillar fcrewed into the Foot, and the 2d a fquare Piece of Brass screwed into the Capital of the Pillar. (3.) Two Pieces of Brafs, moveable on the fquare Part of the Stem, the Lowermost of which is fixed by a Screw in the Side, while the other remains moveable by an adjufting Screw on the back Part. (4.) To the upper Piece, the Stage is adapted by a Dovetail Groove, in and upon which Objects are placed, as in the other Forms, to be viewed, and are adjufted to a proper Focus by the Screw behind. (5.) On the Top of the fquare Piece of Brafs, or Stem, is fixed the Head of the Microfcope, which has a flat Piece of Brafs, properly formed, and placed horizontally and parallel to the Stage. (6.) On the under Part of this Plate is a circular Piece of Brafs, containing 6 Glaffes, difpofed in a circular Manner near its Circumference: This Piece is moveable on a Screw-pin in its Center. (7.) On the upper Part of the Head-piece is another circular Piece of Brafs, fixed at the End with a Screw on the Outfide, and a small Hole Hole in the Center, to which the Magnifiers are fuccef fively applied for fhewing the Objects on the Stage below. (8.) The reflecting Speculum, fcrewed on to the Pillar, and moveable on a Joint, illuminates the Object to be viewed.-From all which, you will readily fee how eafy and pleasant it must be to view Objects in this Conftruc tion of a Microscope. There are many other Ways and Methods of applying fingle Glaffes, but none of them fo well deferve your Notice, as the four different Forms I have now defcribed to you :-But I must take another Opportunity of explaining to you, the Difference between fingle and compound Microfcopes; the different Conftruction of the latter,and, laft of all, the Nature and Conftruction of the Solar Microscope, which must be the Subject of our enfuing Microscopic Speculations*. DIALOGUE II. Of the Nature, Conftruction, and Use of COMPOUND MICROSCOPES. A Euphrofyne. Tour laft Interview you entertained me with an Explication of the different Forms and Construction of fingle Microscopes, and promised me, in our next, the fame Thing with regard to compound ones; and I fee, by the Instruments before me, that you intend to be as good as your Word.- -They are equally numerous, and much exceed them in Magnificence of Appearance; but, I fear at the fame Time, fince they are not fo fimple in their Nature as the former, their Conftruction and Effects will not be fo eafy to be comprehended. Cleon. When you fee the Parts of which they are compofed, you will not find them so very complicated as you imagine; and tho' they ftrike the Eye with Grandeur and Sumptuofity, yet the effential Parts, by which the Effects are produced, will be found to be very few, and those easy to be understood. See Plate XLIV. Euphrof. I hope I fhall find it fo; but, pray, is the Effect of a compound Microscope above that of a fingle One, in any Proportion to its Magnitude? if it be, they must be wonderful Inftruments indeed. Cleon. In answer to that Query, my Euphrofyne, I need not inform you, that your Sex, as well as our own, are very apt to be affected with Grandeur and Magnificence, in every Shape; and the Microscope, to many People, as much recommends itself by a pompous Appearance, as by its useful Effects: But as we are only at present concerned with the latter, I fhall proceed to fhew you the Difference between a fingle and compound Microscope, which confifts in the following Particulars: (1.) In the fingle Microscope one Glafs only was neceffary, because the Object itfelf was viewed by that Glafs; but (2.) in a compound Microscope the Image of the Object is viewed by the Eye, and therefore two Glaffes at least are neceffary in this Construction, viz. one to form the Image of an Object, and the other to fhew it magnified. Euphrof. Then I apprehend, by what you have faid, that only two Glaffes are effential to a compound Microfcope: But befides the two which you have now fhewn me, I obferve there are two others in the Inftrument; pray, to what Purpose do they serve? Cleon. They only serve to make Vision more diftin& and pleasant, by properly attempering the Rays of Light, and enlarging the Field of View. The original Conftruction was with the two Glaffes only; then a 3d was added, for an Improvement of the Invention; and, at laft, a 4th, to compleat that Improvement.-It requires a mathematical Skill in Optics, to conceive the Rationale of them all; but you will eafily find, by Experiment, that they have each of them their peculiar Ufes, in producing the great Effect of a manifying Power in the best Manner poffible. Euphrof. You will be fo good as to point out to me the Ufes of each particular Glafs, and illuftrate the fame by an Example. Cleon. That I fhall readily do, with an Object a little in your own Way, I mean, a Row of the smallest Minikin Pins, which you can give me; and you will be no lefs pleased with the Oddity of their Appearance, than you are with their Ufes in Dreffing. |