178 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY.

bending of the ray a a, in passing through them(Fig. 80).It will be evident that no greatdifficulty exists in measuring therefractive powers of different trans-parent bodies: and that hence weare enabled to tabulate those whichhave the highest and lowest refrac-tive indices. A few of the mostimportant are given in the following

30. table:—Al, he ea aan 1:000294:Mater... vo... iv. ivnnens 1-336Alcohol 5. 0 ih sh ae 1:372ON of cloves ines.vis nie us a 1-535Cromn.glass....:.000......000, 1-534:Phiegles..,................., 1-542Bintglsy.........c0......... 1:830Do. containing much lead...... 2-028Diomond, ons.oiinin sss on 2-439

This knowledge enables us to trace a ray of light throughtransparent bodies of any form, provided we can find the incli-nation of the incident ray to the surface, where it either entersor quits the body.

If parallel rays fall upon a plane surface ¢, of glass, they willretain their parallelism after passing through it as the ray a(Fig. 81). The rays diverging from the point A, will be refractedby the first surface into thedirections& 6, and by con-tinuing ¢ a, and& 8, back-wards, we shall find theymeet at a point beyond A:so that supposing the eyeto be placed within thebody&, the point A wouldappear removed to B. Butwhen the rays undergo a second refraction by passing out ofthe second surface, we shall find by continuing the lines back-

"wards that they meet at ¢; therefore a plane glass diminishes

the apparent distance of the point of the diverging rays. If,instead of a plane glass, we employ a piece equally curved,like a watch-glass, it produces very little change in the formand position of objects.

Lenses are glasses ground to different forms, their surfacesbeing segments of spheres, and it is in obedience to the refrac-