The smoke from brown paper is very useful in showing the various directions of the rays of light when reflected from plane, convex, and concave surfaces. The equal angles of the incident and reflected rays may be perfectly shown by using the next arrangement of apparatus. (Fig. 276.) [Illustration: Fig. 276. A. Rays of light slightly divergent issuing from the lantern, and received on a little concave mirror, which brings the rays almost parallel, and reflects them to E, a piece of looking-glass, from which they are again reflected. C is the incident, and D the reflected rays. F. Smoke from brown paper.] A very dense white smoke is obtained by boiling in separate flasks (the necks of which are brought close together) solutions of ammonia and hydrochloric acid. The opposite properties of convex and concave mirrors--the former scattering and the latter collecting the rays of light which fall upon them--are also effectively demonstrated by the help of the same illuminating [Page 288] source and proper mirrors, the smoke tracing out perfectly the direction of the rays of light. (Fig. 277.) [Illustration: Fig. 277. The smoke shows the rays of light falling on a convex mirror, and rendered still more divergent.] The smoke developes the cone of rays reflected from a concave mirror in the most beautiful manner, and by producing plenty of [Page 289] smoke, and turning the mirror about--the position of the focus (_focus_, a fire-place), is indicated by a brilliant spot of light, and the reason the images of objects reflected by the concave mirror are reversed, may be better understood by observing how the rays cross each other at that point. (Fig. 278.) [Illustration: Fig. 278. The smoke shows rays of light falling on the concave mirror. In this experiment attention should be directed to the bright point, E, the focus where the convergent rays meet.] One of the most perfect applications of the reflection of light is shown in the "Gregorian reflecting telescope," or in that magnificent instrument constructed by Lord Rosse, at Parsonstown, in Ireland. (Fig. 279.) [Illustration: Fig. 279. Lord Rosse's gigantic telescope.] The description of nearly all elaborate optical instruments is somewhat tedious, but we venture to give one diagram, with the explanation of the Gregorian reflecting telescope. (Fig. 280.) [Illustration: Fig. 280. The Gregorian reflecting telescope.]