784 



• PLATE XXVII. 



Fig. 375. A is an Actual focus of diverging rays, B 

 M actual focus both of couverging and of diverging rays, 

 C a virtual focus of converging rays, and D a virtual 

 focus of diverging rays; A and B, B and C, and C and 

 D are foci conjugate to each other, with respect to the 

 refractions of the three lenses. P. 415. 



Fig. 376. The image of the point N, formed by the 

 plane mirror A B, is at an equal distance behind the 

 mirror; and in this manner the whole image of the 

 word is formed in an inverted position. P. 415. 



Fig. S77. ABCD represents a pencil of parallfl 

 rays falling on the concave mirror C D, and collected 

 into the principal focus at E, wWch is half way be- 

 tween the surface and its centre. F is tlie principal 

 focus of the convex mirror G; and H that of the re- 

 fracting surface I. P. 416. 



Fig. 378. A being the centre of the concave mirror 

 B, tlie image of an object at C will be found at D, and 

 the reverse. P. 416. 



Fig. 379. A pencil of light, deflected from its path 

 by a prism of a denser substaiicc, in different posi- 

 tions. P. 416. 



Fig. 380. A pencil of light scattered into various 

 directions by a multiplying glass. P. 416. 



Fig. 381. A is a section of a double con vex lens, B of 

 a double conc«ve. C is a planoconvex, D a planocon- 

 cave; and E and V meniscus lenses; but a menis- 

 cus ot the form represented by F is sometimes called 

 a concavoconvex lens. P. 417. 



Fig. S8'.J. The pencils of light A, B are refracted by 

 the convex lens tj in the same manner as lliey would 

 have been by the circumscribed double prism DE; 

 and in the same manner the concave lens F resembles 

 in its operation the prisms G, H. P. 417. 



Fig. S83. A, a pencil of parallel rays, made to con- 

 verge, by a double convex lens of crown glass, to the 

 centre of cui-vature of one of its surfaces. B a double 

 concave lens, causing the rays to diverge from the 

 centre of curvature. C, D a planoconvex lens, of 

 which the principal focus is at the distance of a dia- 

 meter. P. 417. 



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. 417. 



Fig. 385. At A, a r.idiant 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, tlie other 

 nearer; and CD is to D E as EF to FG; D and F 

 being the principal foci of the lens. P. 418. 



Fig. 386. Thie oblique pencils of rays A, B, and 



the direct pencil C, are supposed to be brought to 

 their respective foci in the same plane D E. P. 419. 



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. 421. 



Fig. 388. The box of Count Rumford's photome- 

 ter. 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 

 EF. The wings of the cylinders serve to make the 

 shadows of equal breadth. The shadows are viewed 

 through the aperture at G. P. 421. 



F'ig. 389. Dr. Wollaston's instrument for the mea- 

 surement of refractive densities. A is a rectangular 

 prism of flint glass, under which the substance to be 

 examined is attached; BC is a rod, or ruler, 10 

 inches lone, C D and D E are each 15 ,»jji. Wlicn the 

 sights at B and C are so placed that the division be- 

 tween 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. 421. 



Fig. 390. A is the actual image of the candle B, 

 formed by the convex lens C. P. 422. 



Fig. 391. A is the actual image of the candle B, 

 formed by the concave mirror C. P. 422. 



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 nom the lens. P. 

 422. 



F'ig. 393. A is the virtual image of the Candle B, 

 placed within the focal distance of the concave mirror 

 C, the image remaining erect. P. 422. 



Fig. 394. A is the virtual image of the candle B, 

 formed by the concave lens C, and less than the ob* 

 ject. P. 422. 



Fig. 395. When the object A is placed in the prin- 

 cipal focus of the convex lens B, a virtual image i% 

 formed at an infinite distance, which subtends, when 

 viewed from C, or from any other point, the same an- 

 gle as the object subtends at the centre of the lens. 

 P. 422. 



Fig. 396. The object A being placed a little within 

 the focus of the lens B, a virtual image C is fonned, at 

 such a distance as is most convenient to the eye, which 

 subtends the same angle a> the object, from theceotrC 

 of the lens, and therefore appears somewhat more 

 magnified than when the object is in the principal fo- 

 cus. P. 422. 



