32 



• KNOWLEDGE 



[July 11, 1884. 



OPTICAL RECREATIONS. 



Bv A Fellow of the Royal Astrosojiical Society. 



(Continued from p. 482.) 



WE stated on p. 307 that light travels in straight lines, 

 but we must now introduce a qualification into thi.s 

 general statement, and add, as lonr/ as it is passing tliroucjh 

 a homogeneous mndium — or one ofahsoluteJy uniform density 

 throughout. This is the way we proceed in physical science. 

 We first enunciate a law which we assume to be universal, 

 and then we deal with what are — or seem to be — exceptions 

 to it ; and ascertain and show whether they are really so, 

 or whether both they and the facts on which our original 

 asstimption was founded may not in reality be susceptible 

 of inclusion in a higher and more general law still. Now 

 while it is perfectly true, as we have just said, that the 

 propagation of light is absolutely rectilinear (or io straight 

 lines) in a medium of uniform density, when it passes from 

 a rarer medium into a denser one, or rice-versA — save in a 

 direction accurately square to the surface bounding such 

 two media — it is bent from its original direction, or, to use 

 the technical term, " refracted." This may be well shown, 

 and an approximation made to the determination of the 

 law of retraction, by the simjile apparatus depicted in 

 Fig. U. 



It is nothing more than a clear glass (18oz ) physic bottle, 

 on the front of which a piece of white pajier is pasted. 

 Prior to stickin? the paper on, an accurate circle uiust be 

 cut out of it ; and, when all is dry, two thick diametrical 

 lines, P, P' H H', may be ruled in ink on the glass, the first 

 vertical and the second truly square to it, of course horizon- 

 tal. Or, if preferred, two pieces of stout filk cord may be 

 gummed across the aperture. A piece of blackened card 

 should be cut out to cover one of the narrower sides of the 

 bottl?, and near the top of this a hole; or, perhajis, pre- 

 ferably a horizontal slit should be cut. The slit must be a 

 very narrow one. We must now fill our bottle exactly up 

 to the line H, H' with water, j^lst coloured with a little 

 milk, and, immediately before beginning our experiment, 

 pufl" a mouthful of cigar smoke into the mouth of the 

 bottle. We employ the milk and the smoke in order that 

 we may see our lieam of light, for the student will recol- 

 lect (p. 30G) that light passing in a direction across 



our line of sight is absolutely invisible. Very well, 

 then, we have next to obtain our beam of light ; and, 

 undoubtedly, for the majority of ojitical experiments, direct 

 sunlight is the best. In the present case, though, this 

 involves the use of a darkened loom with a narrow beatu. 

 of sunlight admitteii through a hole in the shutter. If the 

 reader has any apartment at his dis^posal which he can so 

 employ, and which is sufficiently light-tight, so much the 

 better ; if not, he may use a good kerosine lamp and a. 

 buU'seye condenser, and conduct his investigation at night. 

 Whichever source of light he employs, he must, by the aid 

 of a looking-glass or otherwise, throw the beam proceeding 

 from it through the slit in the blackened card at the side 

 of the bottle, shifting the mirror about until the line of 

 light strikes the surface of the water at C. Our puff of 

 tobacco smoke will enable him to do this. Now trace the 

 course of the light through the slightly tuibid water, and 

 carefully note the path it takes. Is no longer preserves its 

 original direction, but is bent towards PF ; in other 

 words, its track becomes more upright. Bear in 

 mind that, coming through the side of the bottle, 

 it has originally struck the surface of the water 

 obliquely. Had we sent it through the mouth of the 

 bottle truly square to the surface of the water it would have 

 preserved its perpendicular direction throughout — much in 

 the same way (though, of course, in one sense, for a diffe- 

 rent reason) that a perpendicular ray which falls on a 

 mirror is reflected accurately back along the line by which 

 it originally travelled. This by the way. To return to our 

 experiment. Note where the beam of light cuts the semi- 

 circle H, P, H', before entering the water ; as ako the point 

 where it crosses the semicircle H, P', H', after having done 

 so, and draw ink lines from these points to the perpen- 

 dicular P, P', taking care that those lines are parallel to 

 H, II'. If we measure these lines carefully with a [lair of 

 compasses we shall find that the one above the water is — 

 as nearly as may be — 1 J times the length of the lower one. 

 And, what is particularly to be remarked, that while the 

 length of these respective lines varies with the obliquity 

 with which the light falls on the surface of the water, the 

 proportion between them remains invariable. Before pro- 

 ceeding to investigate the law of Refraction we may just 

 remark incidentally that, as our exjjeriment has been so far 

 conducted, evidently part of the light has been reflected from 

 the surface of the water, the unreflected portion alone, 

 of course, penetrating it. For our present purpose, how- 

 ever, we may neglect this reflected part altogether, and 

 consider that the whole incident beam enters the water, 

 and is bent at the surface where it does so. By the aid of 

 Fig. 1.3 we may now explain the law in virtue of which 



.1 



Fig. 15. 



light follows a fixed and definite course rn passiag froED 

 any medium into another of different density — a law dis- 

 covered by Sriell during the first quarter erf the seventeenth 



