16 



♦ KNOWLEDGE ♦ 



[Nov. 1, 1885. 



most complex and highest result of inorganic 



and organic evolntion. 

 QnestioE? -unan^cered by Evolution : 

 "Wliat lies beyond the phenomenal. 

 The beginning of matter. 

 The origination of motion. 

 Passage from the action of the brain to facts of 



consciousness. 



Application to that which most concerns man, 

 namelVf human relations. 



OPTICAL RECREATIONS. 

 Bv A Fkllow of the RoTAt AsTROsoiiicAL SociEiy. 



C'OLOl'K AND LIGHT. 



X describing the refraction of a ray of light 

 by means of a prism (Knowledge, Vol. VI., 

 p. 46) Tve incidentally referred to the fact 

 (only, hoi,vever,immediatelyto dismiss it) that 

 the image of any object seen through such a 

 prism was edged with brilliant colours, and 

 that, in fact, white light passing through it 

 was separated into a sheaf of rnys cf different re- 

 frangibilities, or capacity for being bent from a straight 

 path. For the better comprehension li the subject of 

 colour generally, and of the colours of objects in parti- 

 cular, it now becomes necessary to revert to the use of the 

 prism, and to endeavour to ascertain of what so-called 

 white light is composed, and how it comes to pass, for 

 example, that while a geranium is scarlet its leaves are 

 green. 



But befi re proceeding to our preliminarj- experiment, 

 let us for a few moments devote ourselves to the con- 

 sideration of a subject concerning which a good deal has 

 appeared from time to time in our late Correspondence 

 Columns. We have insisted, ovtr and over again, in 

 these papers on the fact that light consists of a series of 

 waves er undulations in some substance which fills the 

 visible universe; and which, for want of a better name, 

 is called the sether. This undulatory motion is purely 

 mechanical ; but the tiny waves, by ultimately beat- 

 ing upon the retina of the eye (Fig. 1) set that in 



■^'^ei^ hum<:u 



CrygtalUne CinsM 

 Cornea - 



Opt'k^Yerrc 



^clerotfc 



FiK. 1. 



the wave. But, and this is the point to be especially 

 noted, until they reach the sensorium, they are mere 

 mechanical vibrations. It is the brain, and that alone, 

 which re'Utlers them as colour. Perhaps this may not be 

 an inappropriate place to add a word, as to a little diiB- 

 culty which may at this point possibly have strtick .some 

 verv careful reader. We refer to the fact of these rays of 

 such varving lengths all reaching the eye at once. Well, 

 the explanation of this is of a sufficiently simple character. 

 In reality, the undulations are not only of various lengths, 

 but travel with various velocities ; the smaller undula- 

 tions travelling more quickly than the larger ones, and 

 so all arriving at the eye at once : just as a crowd of men, 

 weunen, and children might travel in a compact mass if 

 the children took two or three of their little trotting 

 steps to ever}- single stride of the men, and the women 

 timed their pace cerrespondinglj". And now wc will 

 proceed to our experiment. 



turn in vibration, which vibration is communicated 

 bj- means ff the ojilic nerve to the brain. Then, 

 and not until then, do these purely mechanical undula- 

 tions become light, as we know it. Furthermore, the 

 multitude of wiives of which ordinary white light is 

 composed are of the most various lengths. If we suppose 

 that their length is 3-^50*^ o^ f'^i inch, we shall see what 

 we call red ; shorten them to ^r^e^th of an inch, and we 

 have the sensation of green; shorten them still farther 

 to s^Tjo^li cf ^1 inch, and the e\e translates them into 

 violet and so on, the colour varying with the length of 



Fiij. -2. 



Through a holeH (Fig. 2) in theclosed shutter of an other- 

 wise thoroughly darkened room, a beam of sunlight H 6 

 is allowed to enter ; and, of course, as every one knows, 

 will, if uninterrupted, form a round spot of light some- 

 where on the floor. If now, thotigh we turn one side 

 of the prism (seen endways at) P towards the beam of 

 light, it will pass thrnagh the prism, and be pirojectcd on 

 a white screen suitably placed, as a long streak e'f the 

 most gorgeous colours ; those in fact vulgarly known as 

 " the colours of the rainbow." If we suppose our prism 

 to be placeel as in the figure with a sharp edge downwards 

 and its flat base at the top, we .shall note that, beginning 

 at the top, the light is of a dull violet ; this shades into a 

 strong blue ; as does that into a greenish blue, melting 

 in turn into green. Then we arrive at a region of the 

 spectrum (as our strip of coloured light is called), where 

 greenish yellow and orange-yellow intermingle in a way 

 which renders the observer doubtful whether ptire yellow 

 exists at all. As we descend still farthe^r, the orange- 

 yellow becomes a pronounced and elecided orange, and 

 this shades into scarlet, which eleepens into a crimson 

 when the spectrum terminates. It will hence be seen, 

 as before intimated, that the violet raj-s are the most bent 

 from their original path, the crimson ones the least so. 

 We have, as a matter of fact, been repeating Sir Isaac 

 Newton's classical experiment, by the nid rf which he 

 demonstrated the compound nature of light. We may 

 avail ourselves of another device of his in this investiga- 

 tion, a device shown in Fig. 3. 



It enables us to isolate any particular colour, and to 

 examine it apart from the rest, and consists if a narrow 

 slit s in the screen S, letting through light e f one re- 

 frangibility only. If instead of admitting our beam of 

 sunlight throuffh a round hole in cur closed shutter, we 



