DESCRIPTION OF PLATES. xxix 



lines perpendicular to D E at any such distances, that B K may be to B L in a cer- 

 tain proportion, which is that of the sines of the angles of incidence ABM, C B M, 

 to those of the angles of refraction FBN, GBN. BO and B P are the reflected 

 portions of the rays. P. 322. 



Fig. 370. A mode of determining the position of a refracted ray, which is parti- 

 cularly convenient in the case of refractions at spherical surfaces. ABC being any 

 circle, either touching the refractive surface at A, or being itself a section of the 

 refracting substance, if another circle D E F be drawn on the same centre, having its 

 diameter to that of the first as the sine of the angle of incidence to that of refrac- 

 tion, and a third circle G H I, which is less than the first in the same proportion as 

 the second is greater ; and if the direction of the incident ray K A be continued to D, 

 and L D be drawn from the centre cutting G H I in G, A G will be the direction of 

 the refracted ray ; and if this ray pass again out of the denser medium at B, its 

 direction B M may be found by drawing L I F, and F B M will be thus truly deter- 

 mined. P. 322. 



Fig. 371. A ray or pencil AB, refracted at B to C, and there reflected by a 

 perpendicular surface into an opposite direction C B, will return also in the direction 

 B A, a portion of it being reflected in the first place to D, and in the second to E. 

 P. 323. 



Fig. 372. A pencil AB passing through a substance CD contained between 

 parallel surfaces, continues its course in the direction E F parallel to A B. 

 P. 324. 



Fig. 373. The ray A B, entering the medium C D through the transparent sub- 

 stance E F, contained between parallel surfaces, acquires the direction GH, parallel 

 to IK, into which LI is at once refracted. P. 324. 



Fig. 374. The appearance of a prism, of which the lower surface is divided into a 

 bright and a dark portion, separated by a coloured arch ABC. P. 324. 



PLATE XXVII. 



Fig. 375. A is an actual focus of diverging rays, B an actual focus both of con- 

 verging 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. 325. 



Fig. 376. The image of the point N, formed by the plane mirror AB, 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. 325. 



Fig. 377. A BCD represents a pencil of parallel rays falling on the concave 

 mirror C D, and collected into the principal focus at E, which is half way between 

 the surface and its centre. F is the principal focus of the convex mirror G ; and H 

 that of the refracting surface I. P. 326. 



Fig. 378. A being the centre of the concave mirror B, the image of an object at 

 C will be found at D, and the reverse. P. 326. 



Fig. 379. A pencil of light, deflected from its path by a prism of a denser sub- 

 stance, in different positions. P. 326. 



Fig. 380. A pencil of light scattered into various directions by a multiplying 

 glass. P. 326. 



Fig. 381. A is a section of a double convex lens, B of a double concave. C is a 

 planoconvex, D a planoconcave ; and E and F meniscus lenses ; but a meniscus of 

 the form represented by F is sometimes called a concavoconvex lens. P. 326. 



Fig. 382. The pencils of light A, B are refracted by the convex lens C in the 

 same manner as they would have been by the circumscribed double prism D E ; and 

 in the same manner the concave lens F resembles in its operation the prisms G, H. 

 P. 326. 



Fig. 383. A, a pencil of parallel rays, made to converge, by a double convex lens 

 of crown glass, to the centre of curvature of one of its surfaces. B a double con- 





