DESCRIPTION OF PLATES. xxxi 



PLATE XXVIII. 



Fig. 397. An imperfect image of an external object, painted in a dark room, in 

 an inverted poskion, by the light coming in right lines through a small aperture. 

 P. 332. 



Fig. 398. A portable camera obscura. A is a lens, B a mirror placed obliquely, 

 and throwing the image on a plate of ground glass, CD. E is a moveable cover, 

 and F G a screen attached to it, for excluding foreign light. P. 332. 



Fig. 399. A camera obscura, which throws down an image, by means of the 

 mirror A, and the lens 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. 332. 



Fig. 400. An arrangement proposed for a solar microscope, adapted to a window 

 facing the south. The mirror A is moved by a hinge into the position required for 

 the day, and during the employment of the instrument is turned only round the axis 

 A B, which is parallel 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. 333. 



Fig. 401. An arrangement proposed for a phantasmagoria. The light of the 

 lamp A is thrown by the mirror B 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 fixed 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 such a manner, that 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 suffers 

 the screen M to fall and intercept a part of the light. The rod K N must be equal 

 to I K, and the point 1 must be twice the focal length of the lens F, before the object, 

 L being immediately under the focus of the lens. The screen M may have a trian- 

 gular opening, so as to uncover the middle of the lens only, or the light may be inter- 

 cepted in any other manner. P. 334. 



Fig. 402. The construction of the astronomical telescope. ABC and DEC are 

 the central parts of the pencils of rays, coming, from the extremities of the visible 

 field, through the middle of the object glass. P. 334. 



Fig. 403. The extreme pencils of rays in the double or compound microscope. 

 P. 334. 



Fig. 404. The extreme pencils in the Galilean telescope, or opera glass. P. 334. 



Fig. 405. A, the directions of the extreme pencils in the common day telescope of 

 Rheita. If only two eye glasses were employed, as at B, the field would obviously 

 be more contracted. P. 334. 



Fig. 406. Dr. Herschel's forty feet telescope. ABC the path of a ray of light, 

 reflected by the mirror at B to the eye glass C. Da 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 

 is raised or depressed by the pullies at H and I. K one of two rooms or huts for 

 the accommodation of the observer's assistants. The wheels, under the frame, serve 

 to turn the whole instrument round its centre. P. 335. 



Fig. 407. The Newtonian telescope, with the direction of the central rays. 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. 335. 



Fig. 408. The supposed path of the central rays in the Gregorian telescope. 

 P. 335. 



Fig. 409. The supposed path of the central rays in Cassegrain's telescope. Here 

 the rays actually represented would not only be intercepted by the small mirror, but 





