46 



♦ KNOWLEDGE 



[July 18, 1884. 



the -work it has so grandly sustained in the past, in the 

 support of scientific research and the education of the 

 people. — The American Naturalist. 



OPTICAL RECREATIONS. 



By a Fellow of the Royal Astronomical Society. 



{Continued, from page 34.) 



'l^/'E have seen what happens (Fig. 16, p. 33) when 

 » T a ray of light passes from a rarer medium into 

 a denser one, whereof the boundaries are parallel, and 

 out again into the rarer medium. Let us inve.stigate 

 now what will occur if, instead of our plate of, say, glass 

 liaving parallel sides, those sides are incliued to each other. 



Fig. 18. 



Let A B C in Fig. 18 represent the section of such a stalk 

 of glass (known technically as a prism), and bearing in 

 mind the principles enunciated on p. 32, and illustrated in 

 Fig. 1.5, let us trace the course of a ray of light, RP^;E, 

 incident on one face of our prism at P ; then at P we draw 

 the perpendicular to the surface A B, represented by the 

 dotted line. Now it will be seen that on passing from air 

 into glass our ray R P will be bent tovmrth this perpen- 

 dicular, and will travel in the direction Pp. At p we 

 erect another perpendicular, and, as the student by this 

 time must be well aware, the emergent ray will now be 

 bent from this, and will travel in the direction p E. 

 Hence any object, such as a candle-ilame, placed 

 at R, and viewed through a prism with its base 

 downwards, by an eye situated at E will be seen in the 

 direction R'pE. It will not only be seen .to be thus 

 displaced, but its edges will appear to be fringed 

 with the exquisite colours of the rainbow, though with this 

 phenomenon we have no immediately present concern. It 

 has its origin in the fact that ordinary white licht is a 

 compound of coloured lights, and that each of these 

 colours is bent at a different angle, red being the least 

 bent, and violet or la^■ender the most deflected. It is 

 this dispersion of light which lies at the basis of what is 

 called Spectrum Analysis, of which our Editor has an- 

 nounced his intention of himself treating in these columns. 

 For our i)resent purpose we must consider that we are 

 dealing with light all of one colour. Very well then, with 

 this temporary qualification, the angle formed by R P and 

 ;> E is called " the angle of deviation," and the greater 

 the angle between the sides of our prism, or the greater the 

 refracting power of the material it is composed of, the 

 greater will this angle of deviation become. Note here 

 particularly that in whatever position we place the prism, 

 the emergent ray, p E, will deviate towards the thicker 

 part, or base, of the prism B C. Now, everybody knows 

 what a convex or magnifying-glass is, and if we suppose 

 Fig. 19 to represent a section of such a glass, we shall see 

 that, in effect, it consists of two prism?, ABC, D B C, 

 placed base to base, and that parallel rays, R R R, &c., from 

 a distant object will be so bent as all to unite at F, 



the principal focus of the lens, when they will form an 

 image of the object, just as in the case of the 



concave mirror whose action was described and illustrated 

 on p. 436. And here, again, we have an illustration of 

 that law which can never be too often insisted on — viz., 

 that rays of light go and return by the same route, for if 

 we place a very small bright light at F, the rays diverging 

 from it will be rendered parallel by the lens, and emerge in 

 that condition on the other side of it. Suppose, though, that 

 we remove our light to a point outside of the principal 

 focus of the lens, then, instead of the rays issuing from its 

 distal face being parallel, it will be seen that they will be 

 convergent ; in fact, an image of the source of light will be 

 formed on a screen held at a suitable distance on the 

 other side of the lens. Conversely, if the light be shifted 

 to the position occupied by tlie screen, its image will be 

 formed at the point which it occupied before such shifting. 

 These interchangeable points are called the " conjugate 

 foci " of a lens. All this may be compared with the pro- 

 perties of a concave mirror explained on p. 436. It ia 

 evident that, if we obtain a lens whose focal length equals 

 the width of the room shown in Fig. 2 (p. 306), and put 

 this in the place of the simple hole in our shutter, we shall 

 obtain a much more brilliant and distinct image of the 

 external landscape on the wall ; in fact, we shall have con- 

 structed a primitive form of the camera obscura. The 

 form the camera takes, as arranged for public exhibition, is 

 shown in Fig. 20. 



Fig. 20. 



Here we have a light-tight room, R, usually of octagonal or 

 cylindrical shape, containing a table T covered with plaster 

 of Paris or painted with dead-flat white paint. Above is 

 a box turning in a ring containing the convex lens L placed 

 vertically, and behind it the mirror M at an angle of 4.5°, 

 the effect of this :\rrangement obviously being that the 

 image formed by the lens is reflected down on to the table 

 T, where the spectator sees a charming miniature view of 

 the external landscape, with its drifting clouds, running 

 water, and moving forms of animal life. One of the most 

 familiar uses of a convex lens is that illustrated in Fig. 21. 



