PLATE XXVIII. 



Fig. Z9T. An imperfect image of an externa! object, 

 painted in a dark room, in an inverted position, by the 

 light coming in right lines tlirough a small aperture. 

 P. 425. 



Fig. 398. A portable camera obscura. A is a lens, 

 B a mirror placed obliqnely, and throwing the image 

 on a plate of ground glass, CD. E is a moveable cover, 

 and FO a screen attached to it, for excluding foreign 

 light. P. 425. 



Fig. 399. A camera oljscura, whicli throws down an 

 image, by means of the mirror A, and the Ions B, on 

 the surface C, where it may be seen through the aper- 

 ture D. The surface C has here the curvature best 

 adapted to receive every where a perfect image of a 

 distant object. P. 425. 



Fig. 4o6. An arrangement proposed for a solar mi- 

 croscope, adapted to a window facing the souUi. I he 

 mirror A is moved by a hinge into the position required 

 for tlie day, and during the emph)ymeut of the instru- 

 ment is turned only round the axis .\ B, which is pa- 

 rallel to that of the earth. The mirror C is fixed: it 

 receives the beam of light from A, and throws it on the 

 object through the lenses D and E, of which the joint 

 focus is near the magnifying lens F; this lens paints 

 an image of the object in an inverted position on a 

 screen at G. If the focus of the condensing lenses 

 were behind the object, as at H, the light would be 

 liable to be condensed into a spot on the screen at I. 

 P. 426. 



Fig. 401. An arrangement proposed for a phantas- 

 magoria. The light of the lamp A is thrown by the 

 mirror C and the lenses C and D on the painted slider 

 at E, and the magnifier F forms the image on the 

 screen at G. This lens is 6xed to a slider, which may 

 be drawn out of the general support or box H: and 

 when the box is drawn back on its wheels, the rod I 

 K lowers the point K, and by means of the rod K L 

 adjusts the slider in «urh a manner, tliat the image is 

 always distinctly painted on the screen G. When the 

 box advances towards the screen, in order that the 

 images may be diminished and appear to vanish, the 

 support of the lens F sutTers the screen M to fall and 

 intercept a part of the lijht. The rod K N must be 

 equal to I K, and the point I must be twice the focal 

 length of the lens F, before the object, L being iinmc- 

 <liatc!y under the focus of the lens. I'he screen M may 

 have a triangular opening, so as to uncover tlie middle 

 of the lens only, or the light may be intercepted in any 

 otlier manner. P. 427. 



Fig. 402. The construction of the astronomical te- 

 lescope. ABC and D EC arc the central parts of the 

 pencils of rays, coming, from the c:i!tiemities of the visi- 

 ble field, through the middle of the object glass. P. 

 427. 



Fig. 403. The extreme pencils of rays in the double 

 or compound microscoj-e. P..438. 



Fig. 404. The extreme pencils in the Galilean tele- 

 €cope, or opera glass. P. 423. 



Fig. 405. A, ilic directions of the extreme pencils 

 in the common daj telescope of lUicita. If only two 

 €ye glasses were employed, as at H, the field would 

 tibviously be more ccnUactLd. P. 428. 



Fig. 406. Dr. Ilerschers forty fet t telescope. ABC 

 the path of a ray of light, reflected by the mirror at 

 B to the eye glass C. I) a chair in which the observer 

 sits. E a moveable gallery, on which several persons 

 may stand. F G a smooth surface, on which the bottom 

 of the telescope is made to roll along, while its opening 



2 



is raised or depressed by the pullies at II and I. K one 

 of two rooms or huts for the accommodation of the ob- 

 server's assistants. The wheels, under the frame, serve 

 to turn the whole instrument round its centre. P. 429. 



Fig. 407. The Newtonian telescope, with the di- 

 rection of the central r.nys. These are not the rays by 

 which the object is actually seen, because they are 

 intercepted by the small .speculum, but they afford the 

 simplest determination of^ the magnitude of the field 

 of view. P. 429. 



Fig. 408. The supposed path of the central rayl 

 in the Gregorian telescope. P. 429. 



Fig. 409. The supposed path of the central rays in 

 Cassegrain's telescope. Here the rays actually repre- 

 sented would not only be intercepted by the small 

 mirror, but they would also fall on the perforation of 

 the great mirror. They, however, serve equally well 

 to determine the magnitude of the field. P. 429. ■ 



Fig. 410. The supposed path of the central rays ia 

 Dr. Smith's microscope. The rays running directly from 

 the object are intercepted by a screen. P. 429. 



Fig. 411. A; the dotted line represents the curr* 

 called the caustic of a concave mirror, in which the 

 rays proceeding, in the section represented by the fi- 

 gure, from a distant point, would be collected. B; the 

 dotted line is the caustic of a convex mirror. The eye 

 being supposed to be at a great distance from the he- 

 mispherical mirrors C and 13, the images of distant ob- 

 jects ill all directions will be found between the dotted 

 curves, the distance of those curves sliowing the de- 

 gree of confusion. The images of distant objects ia 

 all directions formed by the small concave and convex 

 mirrors E and F, are found between the dotted circle 

 and the straight line touching it. P. 430. 



Fig. 412. The effect of a field glass in a compound 

 microscope; the inner lines showing what would be 

 the magnitude of the field without it. P. 431. 



Fig. 413. The manner in which Mr. Ilamsden em- 

 ployed a planoconvex lens in the eye pieces of his tele- 

 scopes and in his double magnifiers. The curved dot- 

 ted line shows the image of the straight line divided 

 into equal parts, which is formed by the larger lens, ia 

 the focus of the smaller, through which it is viewed. 

 P. 431. 



Fig. 414. An achromatic telescope, with a triple 

 object glass, and with Boscovich's achromatic eve 

 piece, consisting of two similar lenses, one of which is 

 every w.-iy three times as groat as the other, their dis- 

 tance being twice the focal length of the smaller. P. 

 432. 



Fig. 415. The dotted lines AB and CD represent 

 two images of the same object, formed by rays tliffer- 

 ently refrangible, passing through a simple object glass, 

 which are brought, Vjy the effect of the Ions or field glass 

 E, into such places and dimensions aslo subtend nearly 

 the same angle from the eye glass F. P- 432. 



Fig. 410. A represents Mr. Ilamsden's divided eye 

 glass micrometer, the two portions being moved at 

 once in contrary directions by turning the pinion B, 

 until the two extremities of the distance to be mear- 

 sured appear to coincide. P. 433. 



Fig. 417. Dr. Maskelyne's micrometer, made by a 

 double achromatic prism A, exhibiting two images B, 

 C, the different parts of which are made to coincide, by 

 moving the prism backwards and forwards in the direc- 

 tion of the axis of the telescope. Mr. Ramsden thinks 

 that any substance thus interposed must interfere 

 greatly with the perfection of the telescope. P. 4S3. 



