. Chamber's scientific reader : illustrated with wood engravings. Readers. Fig. 29. double will now be easily understood, especially if the general principle with regard to prisms be kept in mind, that rays of light transmitted through them are always refracted to- wards the thick part, be- cause most lenses are simply Thus, take the double-convex and the double-concave lenses—1 and 4 in the figure: the first is as if two prisms were fixed together with their vertices turned outward, and the second the same, only with the vertices of the prisms meeting in the middle. When a ray of lig
. Chamber's scientific reader : illustrated with wood engravings. Readers. Fig. 29. double will now be easily understood, especially if the general principle with regard to prisms be kept in mind, that rays of light transmitted through them are always refracted to- wards the thick part, be- cause most lenses are simply Thus, take the double-convex and the double-concave lenses—1 and 4 in the figure: the first is as if two prisms were fixed together with their vertices turned outward, and the second the same, only with the vertices of the prisms meeting in the middle. When a ray of light, as EI, fig. 30, falls on a convex surface, as AVB, the perpendicular (or normal) at that point, NIC, is the perpen- dicular to the tangent-plane ; and the ray being refracted towards the j)erpendicular, as IF, is there- fore turned towards C, the centre of the curve of the surface. Now, if a ray fall on any other point of AV, since the normal must always be the line from that point to C, and the ray must be refracted towards the nor- mal, it must also be turned towards C ; and so for all rays that fall on AV. In the same manner, all rays that fall on VB would be turned towards C, because they must all be refracted towards the normal at every point, and the normal must always point to C. The effect of the whole surface, AB, then, is to draw rays of light that fall on it together, to a point behind the surface. Rays which draw together in this way are said to converge. This is the effect produced by rays of light which fall on the transparent cornea of the eye ; they are made to converge and pass through the pupil; at least, by means of it, more rays pass through it than if there had been no refracting medium in front of the iris. (Human Physiology, page 73.) This beinir the effect of one convex surface, it is very much greater when there are two together, as in a double-convex lens, fig. 31, which will be at once clear from what was said of the prism. The effect of the pri
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