A Manual Of Photography

178 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. bending of the ray a a, in passing through them (Fig. 30). It will be evident that no great difficulty exists in measuring the refractive powers of different trans- 5 parent bodies: and that hence we are enabled to tabulate those which ^have the highest and lowest refrac- tive indices. A few of the most important are given in the following 30. table:— Air.....................................1-000294 Water...................................1*336 Alcohol........................1-372 Oil of cloves .................1-535 Crown glass ...................1*534 Plate glass....................1-542 Flint glass....................1'830 Do. containing much lead ......2-028 Diamond .......................2-439 This knowledge enables us to trace a ray of light through transparent bodies of any form, provided we can find the incli- nation of the incident ray to the surface, where it either enters or quits the body. If parallel rays fall upon a plane surface G, of glass, they will retain their parallelism after passing through it as the ray A (Fig. 31). The rays diverging from the point A, will be refracted by the first surface into the directions 6 6, and by con- tinuing a a, and 6 6, back- wards, we shall find they meet at a point beyond A: so that supposing the eye to be placed within the body G, the point A would appear removed to B. But when the rays undergo a second refraction by passing out of the second surface, we shall find by continuing the lines back- wards that they meet at c; 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 form and position of objects. Lenses are glasses ground to different forms, their surfaces being segments of spheres, and it is in obedience to the refrac-