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Fig. 7.The solarmicroscqfe.

Of Optics.

object*. but since the image could not bc feen distinctly by the bareeye at a less distance than lix inches, if it be viewed by an eye-glass ef,of one inch focus, it will thereby be brought lix times nearer the eye;and confequently viewed under an angle lix times as large as before ;so that it will be again magnified lix times; that is, lix times by theobject-glafs, and lix times by the eye-glass, which multiplied into oneanother, makes 36 times; and fo much is the object magnified indiameter more than what it appears to the bare eye; and confequently36 times 36, or 1296 times in surface.

But, because the extent or field of view is very small in thismicrolcope, there are generally two eye-glastes placed fometimes dosetogethcr, and fometimes an inch afunder ; by which means, although.the ocject appears lefs magnified, yet the visible area is much enlargedby the interposition of a fecond eye-glals; and confequently a muchpleafanter view is obtained.

The solar microscope, invented by Dr. Lteberkhun , is constructed inthe following mannen. Having procured a very dark room, let a roundhole be made in the window-lhutter, about three inches diameter^through which the fun may cast a cy lind er of rays A A into the room.In this hole, place the end of a tube, containing two convex glassesand an object. viz. i. A convex glafs a a, of about two inches diameter,and three inches focal distance, is to be placed in that end of the tubewhich is put into the hole. 2. The object bb, being put between twoglafles (which must be concave to hold it at liberty) is placed abouttwo inches and a half from the glafs a a. 3. A little more than aquarter of an inch from the object is placed the finali convex glafs cc,whofe focal distance is a quarter of an inch.

The tube may be fo placed, when the fun is iow, that his rays A Amay en ter directly into it: but when he is high, his rays BB must bereflected into the tube by the plane mirrour or looking-glass CC.

Things being thus prepared, the rays that en ter the tube will beconveyed by the glafs a a towards the object bb, by which means itwill be strongly illuminated ; and the rays d which flow from it,through the convex glafs cc, will make a large inverted picture of theobject at D D, which, being received on a white paper, will reprefentthe object magnified in length, in proportion of the distance of thepicture from the glafs cc, to the distance of the object from thefame glafs. Thus, fuppofe the distance of the object from the glafsto be parts of an inch, and the distance of the distinct picture tobe 12 feet or 144 inches, in which there are 1440 tenths of an inch;and this number divided by 3 tenths, gives 4803 which is the

number