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Thus there may be Lens's of various Spheres applied.
1. But theMicrofcofe which is foiindtb anfwer the end beft, is as follows : A b, .fig. 2;. is a round Brafs Tube.whofe' exterior Surface is form'd into a Skrew of a length fome- what let? than the diftance of the Focus of a Glafs convex on both fides, ufed here for illuminating the Objea, and fitted to its Bafe A C, by a Ring with a Strew in it D K.
FG is another Brafs Tube, fomewhat wider than the firft, and open each way for an Objea to be apply'd to the Mxrojnfe. To its upper Bafe G H is faftned a Spring of its Focus Will be Steel-Wire, twifted into a fpiral I ; whereby an Objea apparent one - placed between two round Plates, or Slices K and L, in the manner hereafter mentioned, is by means of the Skrew B C, brought to the Microfcofical Lens, (or magnifying Glafs, whereof there are feveral) and kept firm in its place. Tothe-Bafis H G, nhich has a female Skrew M, are fitted Cells N, with a male Skrew O, wherein Lens's of various Spheres, guarded by Ferrils, are included. In
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they'll have Spots ; fo they muil be drawn out immedi- ately after they are turn'd round. As to the Stem, break it oft asneartheBallaspoffible ; and lodgingthe remainder of the Stem between the Plates ; by drilling the Hole exaclly round, all the Protuberances are buried between the Plates ; and the Mcrofcope performs to admiration.
After thefe manners may spheres be made much fmaller than any Lens ; fo that the belt Micro/cope,, or thofe which magnify the mod, are made thereof. For fuppofe the Diameter of a_ Spherule to be 7 V of a Digit, the diftance of i and therefore its real Diameter to its + s T 4 ; that is, as ,-•, to 8, or as 3 to 511 ; or lattly, as 1 to 170. Its Surface therefore will be mcreafed in the Proportion of 1 to aSjco, and its Bulk in a Ratio of 1 to 4915000.
M. Leewenboeck and M. Muffcbenbroeck have fucceeded
very well in fpherical Micro/copes ; and the Apparatus of the
latter is much commended : But we forbear any Defcrip-
j being eafy for any who confulers the
tions thereof;
Pis a female Skrew, by which an Ivory Handle P Q_ is Structure of thofe confiding of Lens's ,
faftned to the Urcofcope. thofe of Spheres may fee JJj^H?? s ' concuve how
Water Mi croscope. Mr.5. Gray, and after him, W'olfius and others, have contrived Water-Microfcopes, confiding of Spherules, or Lens's of Water inftead of Glafs, fitted up fomewhat after the manner of thofe above-mentioned ; (as Spheres of Water may be likewife ufed inftead of Glafs many of the common Mcrofcofes) But fince the diftance of the Focus of a Lens or Sphere of Water, is greater 'hen in one of Glafs, (the Spheres, whereof they arc Segments being the fame) Water-Microfcopes magnify lefs, and are therefore lefs. efteemed than thofe of Glafs.
The fame Mr. Gray firit obferved that a fmall Drop or Hemifpherule of Water held to the Eye by Candle- Light or Moon-Light, without any other Apparatus, mag- nified the Animalcule contain 'd in it, vaftly more than any other Mcrofcope. The Reafun is, that the Rays conrn' from the interior Surface of the firft Hemifphcre, are re- flefled foastofall under the fame Angle on the'Surface of the hind Hemifphere, to which the 'Eye is applied as if they came from the Focus of the Spherale ; whence they are propagated to the Eye in the fame manner as if the Objects wcte placed without the Spherule in its Focus
Hollow Glais Spheres, of the Diameter of about half a Digit, fill'd with Spirit of Wine, are frequently ufed for Microfcopes j but they don't magnify near fo much.
Theory of Compound, or double Microscopes
Suppofe an Objea-Glafs E D, % a 4 . ffc Segment of a very fmall Sphere, and the Objea A B placed without the Focus F.
Suppofe an Eye-Glafs G H, convex on both fides, and the Segment of a Sphere greater (tho' not too great) than that of D E, and let it be fo difpofed behind the ObieS asthatif CE: CL : : CL : C K, the Focus of the Eve- Glafs may be in K.
Laftly, fuppofe L K : L M: : L M : L I.
If then O be the place wherein an Objea is feen diftina with the naked Eye ; the Eye in this Cafe being placed in I, will fee the Objea A Bdirtinflly in an inverted fituation and magnified in a compound Ratio of M K to L H and L C to C O ; as is ptoved from the Laws of Dioptrics
Microfcopi In the Ivory Slice T are round Holes, in which are fitted little Circles of Mufcozy Glafs, for Objeas, efpe- cially fmall and pellucid ones, as little Infeas, or the Wings, Scales, Sic of larger, to be faftned to.
When live Infeas are to be view'd, they are cover'd with the BrafsSlice Y, which is put in a little fquare Erafs-bed, perforated with Holes X : And the fame Slice, whether alone, or inclofed in the bed, being laid between the round Plates K and L, is brought to the Lens by means of the Screw A B, till the Objea may be diftinaiy view'd.
If other pellucid oblong Objeas are to be view'd, as Down, Cuticle, i£c. inftead of the Slice above, is ufed the Inftrument mentioned above, for viewing Winos of Flies ; whofe Struaure is manifeft from infpedtion.
There are other Inftruments in the Apparatus of the Miaojcope, as little Tongs, &c. for taking up fmall Ob- jeas, a Glafs-Tube for viewing the Circulation of the Blood in Fifties, S^. which need no defcription.
What has been faid hitherto, is to be underftood of Lenticular tScrofapei ; for Spherical ones, their Doarine will be underftood from what follows.
If an Objea A B be placed in the Focus of a Glafs Spherule F, and the Eye be behind it, v. g. in the Focus G; the Objea will be feen diftina, in an erea Situation, and magnified, as to its Diameter, in a Ratio of J of the Dia- meter E I, to the diftance at which Objeas are to be placed to be feen diftinaiy with the naked Eye.
The firft part of the Propofition is proved in the fame manner of Spheres, as of Lens's : As, then, a good Eye fees an Objea diftinaiy at the diftance of 8 Digits, a Glafs Spherule will inlarge the Diameter of an Objea in a Raiio of i of the Diameter to 88 Digits. Suppofe then the Diametet of the Spherule E I T " 5 of a Digit, C E will be =^r, and FE = -,'- ; and therefore F C = ^-)- 4 'j= ,V Confequenfiy, the true Diameter of an Objea to its apparent one is in the Ratio of ,'_ to 80 ; i. e. as 3 to 320, or 1 to ic7 nearly.
Now a Lens convex on both fides, increafes the Diameter in a Ratio of the Semi-diameter to the fpace of 8 Digits ; wher.lote i having a lefs Ratio to 8 than $ ; of a Lens and a Sphere that have the fame Diameter, the former will magnify more than the latter : And pretty much after the fame manner it may be fhewn that a Sphere of a lefs Diameter, magnifies more than another of a laree one. 5
For the Methods of caflhtg little glafs Spherules for Mi- croscopes ; there are various. W'olfius defcribes one as follows : A fmall piece of very fine Glafs, flicking to the wet Point of a Steel-Needle, is to be apply'd to the cxtteme bluifh part of the Flame of a Torch ; or, which is better to the Flame of Spirit of Wine, to prevent its being blacken d. Being there melted, and run into a little round drop, ,t is to be removed from the Flame ; upon which ,t mfiantly ceafes to be fluid : folding then, a thin Hate of Brafs, and making a very fmall fmooth perforation, fo as not ro leave any Roughnefs on the Surfaces ; and further, fmoothing them over ro pre- vent any glaring: fit the Spherule between the Plates a gainfl the Apertures and the whole in a Frame, with Ob- jeas convenient tor Obfervation.
Dr.^to* tells us another Method, thus: Take a piece of fine Wmdow-Glafs, and rafe it with a Diamond into as many lengths as you think needful, 'not exceeding an eighth of an Inch in breadth ; then holding one of thofe lengths between the Fore-finger and Thumb of each Hand over a very fine Flame till the Glafs begins to foften draw it out till it be as fine as an Hair, and break • then' applying each of the ends into the pureft part of the Flame' \ouhave two Spheres prefently, which you may male larger or lefs at pleafure. If they flay long in the Flame
Lams of double Microscopes.
I. ThemoreanObjeais magnified by the Ucrofcope the lefs is ns field, ,. e. the lefs it takes in at one view '
nw J°r-i V ? Eye-Glafs, may be fucceffively apply'd O^-Glaffes of various Spheres; roas,h » both the entire Objeas, but lefs magnified, and their feveral parts much more magnified may be viewed thro' the fame JfaU. n which Cafe, by reafon of the different diftance of the Image, the Tube L K, in which the Lens's are fitted fhoultl be made to draw out. '
For the Proportion of the Objea-Glafs to the Eye- Glafs feme commend the fubduple Ratio, and fome the fubfefquifextile De-Cba teMlt have the Semi-diameter of the Convexity of the Objea-Glafs ,0 be | of a Digit ; or a moft^; in the Eye-Glafs an entire Digit, or even 1 . Cherubm makes the Semi-diameter of the Obiefl-Glafs i, i, <"• jof aDigit; the Semi-diameter of the Eve-Gkr* 1 J, or 1 4 of a Digit. ;
Since 'tis proved, that the diftance of the Image LK from the Objea Glafs D E will be greater, if fnother Lens, concave on both fides, be placed before its Focus ; it follows that the Objea will be magnified the more if luch a Lens be here placed between the Obiect-Glafs D F and the Eye-Glafs G H. Such a Mcrofcope is much con,! mended by Conrade who ufed an Object-Lens, both fides, whofe Semi-diameter was two Digit, j ts A „ r ture equal to a Muftard-Seed ; a Lens concave on both fides la, or at molt is Digits; and an Eye-Glafs convex on b ta fideSj of 6 Digits.
4- Since
