xxx DESCRIPTION OF PLATES. 



cave lens, causing the rays to diverge from the centre of curvature. C, D a plano- 

 convex lens, of which the principal focus is at the distance of a diameter. P. 326. 



Fig. 384. The lenses represented by the shaded surfaces are equivalent in their 

 effects to those of which the sections are shown by the dotted lines ; the figures at A 

 and B being of equal thickness in the middle, and at C at the edges also. P. 326. 



Fig. 385. At A, a radiant point and its image are both situated at the distance of 

 twice the focal length from the lens ; at B, the one is more remote, the other nearer ; 

 and C D is to D E as E F to F G ; D and F being the principal foci of the lens. 

 P. 327. 



Fig. 386. The oblique pencils of rays A, B, and the direct pencil C, are sup- 

 posed to be brought to their respective foci in the same plane D E. P. 327. 



Fig. 387. The square A intercepts the whole light, proceeding from the point B, 

 which would fall on the surface C D, four times as great, placed at a double 

 distance. P. 329. 



Fig. 388. The box of Count Rumford's photometer. The lights, being placed 

 at proper distances on the graduated arms or tables A, B, throw equally dark 

 shadows of the cylinders C, D on a white surface at E F. The wings of the cylinders 

 serve to make the shadows of equal breadth. The shadows are viewed through the 

 aperture at G. P. 329. 



Fig. 389. Dr. Wollaston's instrument for the measurement of refractive densi- 

 ties. A is a rectangular prism of flint glass, under which the substance to be exa- 

 mined is attached ; BC is a rod, or ruler, 10 inches long, CD and DE are each 

 15fgg. When the sights at B and C are so placed that the division between the light 

 and dark portion of the lower surface of the prism is seen through them, the rod F, 

 which carries a vernier, shows the index of the refractive density, which, in the 

 situation here represented, would be 1.43. P. 329. 



Fig. 390. A is the actual image of the candle B, formed by the convex lens C. 

 P. 330. 



Fig. 391. A is the actual image of the candle B, formed by the concave mirror 

 C. P. 330. 



Fig. 392. A is the actual image of the candle B, formed by the convex lens C, 

 being as much larger than the object as it is more distant from the lens. P. 330. 



Fig. 393. A is the virtual image of the candle B, placed within the focal dis- 

 tance of the concave mirror C, the image remaining erect. P. 330. 



Fig. 394. A is the virtual image of the candle B, formed by the concave lens C, 

 and less than the object. P. 330. 



Fig. 395. When the object A is placed in the principal focus of the convex lens 

 B, a virtual image is formed at an infinite distance, which subtends, when viewed 

 from C, or from any other point, the same angle as the object subtends at the centre 

 of the lens. P. 330. 



Fig. 396. The object A being placed a little within the focus of the lens B, a 

 virtual image C is formed, at such a distance as is most convenient to the eye, 

 which subtends the same angle as the object, from the centre of the lens, and there- 

 fore appears somewhat more magnified than when the object is in the principal 

 focus. P. 330. 



