Every dense transparent substance has this ability to yield images by total reflection, each substance having a critical angle of its own; we have just seen that for water this angle is 48° 35´. Glass is made in many varieties, each with a special critical angle, never much different from that of water. A right-angled prism of glass, which any optician can supply, serves as a capital mirror for rays striking its surface at ninety degrees. Such prisms are employed in opera glasses, in hand telescopes, in reflectors for light-houses, and in the Holophane globes we are about to examine. The efficiency of these prisms may be as much as 92 per cent., whereas that of the best silvered mirrors never exceeds 90 per cent. The loss in a prism is due to a slight reflection by the surface on which the rays first fall, and by the absorption of light in the glass itself; this second loss, of course, increases with the thickness of the prism. [Illustration: AB water level. F, G, H, L are refracted to C, D, E, B. I is totally reflected to I.] [Illustration: Holophane globe, vertical section.] [Illustration: Section of Holophane globe. Ray A is refracted as A´, C as C´. B, totally reflected, then refracted, emerges as B´. D takes a similar course, emerging as D´.] Total Reflection in Artificial Lighting: Holophane Globes. Now that we understand the principle of total reflection, let us see how it is applied to increasing the effectiveness of a Welsbach mantle or an electric lamp. And first let us say that we may wish light upon a small area, mainly in a single direction, as downward upon a desk or reading-chair. Or, in a quite different manner, if we are to illuminate a wide space such as that of a large parlor. These requirements are fulfilled by the Holophane globes, devised by M. Blondel and M. Psaroudaki, which are made in many shapes, each adapted to a specific duty. The upper half of each globe is formed into prisms of such angles that, zone by zone, the glass totally reflects impinging rays in just the directions desired. The contouring is accurate to the thousandth part of an inch. With this thorough reflection is combined diffusion as thorough, the interior of the globe being shaped as ribs. Thus, with the least possible waste, the upper half of the source of light is utilized. What of the lower half? Its rays pass through prisms formed so as to refract impinging light into desired paths with but little loss. As a whole, therefore, these globes furnish a beautiful means of illumination with all but perfect economy, special forms of them sending light in any direction desired. [Illustration: Diffusing curves. Holophane globe. Rays are split into b, e, reflected, then as e, f, g, refracted; and into b, c, d, refracted.] [Illustration: Class A, Holophane globe, throwing rays mainly downward. Class B, rays mainly directed at an angle of 60°. Class C, casting rays chiefly in a lateral direction.]