Angle with the Ruler AB. The bottom of this Pipe V was stop'd with a small piece of exactly plain Glass, which was plac'd exactly perpendicular to the Line of direction, or Axis of the Ruler EF. The Pins also TT were drill'd with small holes through the Axis, and through those holes was stretcht and fastned a small Wire. There was likewise a small Pipe of Tin loosly put on upon the end of V, and reaching down to the sight G; the use of which was only to keep any false Rayes of light from passing through the bottom of V, and only admitting such to pass as pierced through the sight G: All things being placed together in the manner describ'd in the Figure; that is, the Ruler AB being fixt perpendicular, I fill'd the Box CC with Water, or any other Liquor, whose refraction I intended to try, till the Wire passing through the middle of it were just covered: then I moved and fixt the Ruler FE at any assignable Angle, and placed the flame of a Candle just against the sight G; and looking through the sight I, I moved the Ruler RS to and fro, till I perceived the light passing through G to be covered, as 'twere, or divided by the dark Wire passing through PP: then turning the Screw in K, I fixt it in that posture: And through the hole S, I observed what degree and part of it was cut by the cross threads in S. And this gave me the Angle of Inclination, APS answering to the Angle of Refraction BPE: for the surface of the Liquor in the Box will be alwayes horizontal, and consequently AB will be a perpendicular to it; the Angle therefore APS will measure, or be the Angle of Inclination in the Liquor; next EPB must be the Angle of Refraction, for the Ray that passes through the sight G, passes also perpendicularly through the Glass Diaphragme at F, and consequently also perpendicularly through the lower surface of the Liquor contiguous to the Glass, and therefore suffers no refraction till it meet with the horizontal surface of the Liquor in CC, which is determined by the two Angles.
By means of this Instrument I can with little trouble, and a very small quantity of any Liquor, examine, most accurately, the refraction of it not only for one inclination, but for all; and thereby am inabled to make very accurate Tables; several of which I have also experimentally made, and find, that Oyl of Turpentine has a much greater Refraction then Spirit of Wine, though it be lighter; and that Spirit of Wine has a greater Refraction then Water, though it be lighter also; but that salt Water also has a greater Refraction then fresh, though it be heavier: but Allum water has a less refraction then common Water, though heavier also. So that it seems, as to the refraction made in a Liquor, the specifick gravity is of no efficacy. By this I have also found that look what proportion the Sine of the Angle of one Inclination has to the Sine of the Angle of Refraction, correspondent to it, the same proportion have all the Sines of other Inclinations to the Sines of their appropriate Refractions.
My way for measuring how much a Glass magnifies an Object, plac'd at a convenient distance from my eye, is this. Having rectifi'd the Microscope, to see the desir'd Object through it very distinctly, at the same time that I look upon the Object through the Glass with one eye, I look upon other Objects at the same distance with my other bare eye; by which means I am able, by the help of a Ruler divided into inches and small parts, and laid on the Pedestal of the Microscope, to cast, as it were, the magnifi'd appearance of the Object upon the Ruler, and thereby exactly to measure the Diameter it appears of through the Glass, which being compar'd with the Diameter it appears of to the naked eye, will easily afford the quantity of its magnifying.
Schem. 1. Fig. 3.
The Microscope, which for the most part I made use of, was shap'd much like that in the sixth Figure of the first Scheme, the Tube being for the most part not above six or seven inches long, though, by reason it had four Drawers, it could very much be lengthened, as occasion required; this was contriv'd with three Glasses; a small Object Glass at A, a thinner Eye Glass about B, and a very deep one about C: this I made use of only when I had occasion to see much of an Object at once; the middle Glass conveying a very great company of radiating Pencils, which would go another way, and throwing them upon the deep Eye Glass. But when ever I had occasion to examine the small parts of a Body more accurately, I took out the middle Glass, and only made use of one Eye Glass with the Object Glass, for always the fewer the Refractions are, the more bright and clear the Object appears. And therefore 'tis not to be doubted, but could we make a Microscope to have one only refraction, it would, cæteris paribus, far excel any other that had a greater number. And hence it is, that if you take a very clear piece of a broken Venice Glass, and in a Lamp draw it out into very small hairs or threads, then holding the ends of these threads in the flame, till they melt and run into a small round Globul, or drop, which will hang at the end of the thread; and if further you stick several of these upon the end of a stick with a little sealing Wax, so as that the threads stand upwards, and then on a Whetstone first grind off a good part of them, and afterward on a smooth Metal plate, with a little Tripoly, rub them till they come to be very smooth; if one of these be fixt with a little soft Wax against a small needle hole, prick'd through a thin Plate of Brass, Lead, Pewter, or any other Metal, and an Object, plac'd very near, be look'd at through it, it will both magnifie and make some Objects more distinct then any of the great Microscopes. But because these, though exceeding easily made, are yet very troublesome to be us'd, because of their smallness, and the nearness of the Object; therefore to prevent both these, and yet have only two Refractions, I Schem. 1. Fig. 4. provided me a Tube of Brass, shap'd much like that in the fourth Figure of the first Scheme; into the smaller end of this I fixt with Wax a good plano convex Object Glass, with the convex side towards the Object, and into the bigger end I fixt also with wax a pretty large plano Convex Glass, with the convex side towards my eye, then by means of the small hole by the side, I fill'd the intermediate space between these two Glasses with very clear Water, and with a Screw stopp'd it in; then putting on a Cell for the Eye, I could perceive an Object more bright then I could when the intermediate space was only fill'd with Air, but this, for other inconveniences, I made but little use of.
Schem. 1. Fig. 3.
My way for fixing both the Glass and Object to the Pedestal most conveniently was thus: Upon one side of a round Pedestal AB, in the sixth Figure of the first Scheme, was fixt a small Pillar CC, on this was fitted a small Iron Arm D, which could be mov'd up and down, and fixt in any part of the Pillar, by means of a small Screw E; on the end of this Arm was a small Ball fitted into a kind of socket F, made in the side of the Brass Ring G, through which the small end of the Tube was screw'd; by means of which contrivance I could place and fix the Tube in what posture I desir'd (which for many Observations was exceeding necessary) and adjusten it most exactly to any Object.
For placing the Object, I made this contrivance; upon the end of a small brass Link or Staple HH, I so fastned a round Plate II, that it might be turn'd round upon its Center K, and going pretty stiff, would stand fixt in any posture it was set; on the side of this was fixt a small Pillar P, about three quarters of an inch high, and through the top of this was thrust a small Iron pin M, whose top just stood over the Center of the Plate; on this top I fixt a small Object, and by means of these contrivances I was able to turn it into all kind of positions, both to my Eye and the Light; for by moving round the small Plate on its center, could move it one way, and by turning the Pin M, I could move it another way, and this without stirring the Glass at all, or at least but very little; the Plate likewise I could move to and fro to any part of the Pedestal (which in many cases was very convenient) and fix it also in any Position, by means of a Nut N, which was screw'd on upon the lower part of the Pillar CC. All the other Contrivances are obvious enough from the draught, and will need no description.
Now though this were the Instrument I made most use of, yet I have made several other Tryals with other kinds of Microscopes, which both for matter and form were very different from common spherical Glasses. I have made a Microscope with one piece of Glass, both whose surfaces were plains. I have made another only with a plano concave, without any kind of reflection, divers also by means of reflection. I have made others of Waters, Gums, Resins, Salts, Arsenick, Oyls, and with divers other mixtures
