320 



♦ KNOWLEDGE * 



[April 17, 1885. 



experiment. It is illustrated in Fig. 39. Here S is the 

 shutter of a room which we have totally darkened. In this 

 shutter is pierced a small hole, h, through which the sun 

 shines. We cover this with a piece of red glass, )•, g. 

 because if we use white, instead of monochromatic light, 

 our shadows will be fringed with the go-called " rainbow " 

 or prismatic colours, and for our present purpose we wish 

 to exclude chromatic phenomena. Very well, then, our 

 narrow beam of red light passes through a condensing 

 lens, L, of short focal length, and is brought to a focus at 

 f, from the brilliant point formed at which the rays 



diverge as seen in the figure. Beyond the focus, say 

 at e, is placed a sharp opaque edge (a new carving- 

 knife with a straight edge answers well) so as to 

 cut off part of the luminous cone, the remaining portion 

 travelling onwards, and being intercepted on the white 

 screen, 8 c, seen edgeways in the figure, S c' to the right of 

 it showing a front view of it. Now, it might be expected 

 that within the limits of the sheaf of rays falling on the 

 screen, the whole space would be equally illuminated ; but, 

 instead of this, it will be seen to be crossed by a series of 

 light and dark bands parallel to the edge of the carving- 

 knife ; most sharply, though not quite sharply, defined as 

 they approach the geometrical shadow, and getting less 

 defined and fainter as they recede from it. If for our red 

 glass we substitute gi-een, the fringes get narrower, and 

 with blue narrower still. As we have, so far, said nothing 

 about the distinction between rays of various degrees of re- 

 frangibility, it must suflice simply to mention this fact here. 

 If, instead of our knife-edge, we interpose a very fine wire 

 iu the cone of rays diverging from /, we shall get fringes 

 outside of the geometrical shadow as before ; but we shall 

 also get a series of very narrow light and dark bands within 

 the geometrical shadow itself. A small circular disc, cut 

 out of tin, and interposed in the course of the rays, gives 

 a bright spot on the screen, surrounded by concentric rings. 

 With white light these rings are beautifully coloured. Solar 

 and lunar halos belong to this class of phenomena, and are 

 caused by particles of mist diffracting the light. These may 

 be imitated, too, by strewing lycopodium powder on glass 

 and regarding any bright light thi-ough it. 



But we may produce these beautiful effects without any 

 dark room at all ; in fact, it we take two thin black cards, 

 and in each of them cut a very narrow slit. Then, by 

 lighting one of these slits by direct sunlight and viewing it 

 through the other, we shall observe the effect shown in 

 Fig. 40. Here, again, though, if we wish to exclude 

 chromatic fringing, we ought to have a piece of red glass 

 at the back of card No. 1, so that the sun passing through 

 it shall give a red line for our examination. 



The possessor of a telescope will be able to produce the 

 most curious diffraction eflTects by the aid of variously 

 shaped diaphragms placed over his object-glass. Fig. 41 



exhibits some of these effects, obtained by viewing Regulus 

 through a series of diaphragms placed over a 4-in. object- 

 glass with a power of 160. A shows the result when the 

 light passed through the opening formed by two concentric 

 squares ; B, that obtained when the aperture was triangular. 

 C, the effect with a series of five small equilateral triangles 



Fig. 40. 



with their apices towards the centre of the diaphragm. D, 

 the appearance afforded by a rhomboidal opening ; while, 

 finally, E shows what was seen when the object-glass was 

 covered with that perforated card on which ladies work 

 book-markers. The most familiar of all diffraction pheno- 

 mena to the observer with the telescope — we mean, of 

 course, the rings surrounding a bright star when viewed 

 with a high power — need no illustration here. They will 

 be found depicted on p. 201 of our first volume. 



Fig. -il. 



Cognate effects to some of those which have been de 

 scribed above may be obtained by placing opposite to /, 

 in Fig. 39, two plane metallic mirrors inclined to each 

 other at a very small angle, or with their surfaces almost 

 in the same plane. These will reflect the diverging beams, 

 so that they meet at a point — or, more rigidly, a line of 

 light — parallel to the edge where the mirrors are in 

 contact ; on either side of which we shall find the bands 

 as before. 



Without going into abstruse mathematical demonstra- 

 tion foreign to the purpose of these essays, it may suffice 

 to point out that, while on the emissive hypothesis of light 

 these beautiful appearances are inexplicable, inasmuch a.s 

 two material particles can never, under any circumstances, 

 annihilate each other, upon the undulatory theory they 

 become intelligible enough. For, as long as the a?ther is 

 still, we have no sensation of light ; but when waves arc 

 set up in the re^thereal ocean and reach our eyes, then does 

 that sensation at once arise. In other words, if our 

 luminiferous ocean is smooth, all is darkness ; when 

 undulations or waves agitate it, there is light. But 



