444 



KNOWLEDGE. 



December, 1913. 



of a different wave-length. In popular lectures, to 

 give an idea of what has happened, I use the simile 

 of a crowd. 



Simile to Picture a Spectrum. 



If a strong light shone on a crowd dressed in all 

 the colours of the rainbow, at a distance the crowd 

 would appear to be clothed in white. 



Suppose the crowd of many thousands to be made 

 up of individuals of all heights, the tallest about 

 twice as high as the shortest. The tallest are 

 dressed in deep red, then as they are shorter the 

 tint gradually becomes crimson, then passes to 

 scarlet, to orange, to yellow. The individuals of 

 intermediate height are yellow-green, still shorter 

 blue-green, then follow all the tints of blue, of indigo 

 and violet down to the lavender-grey, which some eyes 

 can see and others cannot. Colour-blind men 

 might not be able to see the tall ones. Eyes differ 

 in their power to see the two ends of the spectrum. 

 All the crowd exactly keep up with one another. 

 The tall red fellows stalk along, and the 

 little weak violet ones run to keep up, the size 

 of the stride representing the wave - lengths of 

 various coloured light. Every individual proceeds 

 forward in an exactly straight line. Then they 

 come to the prism ; it is a strong turning impulse. 

 Imagine a blast of wind blowing at right angles to 

 the travelling crowd. It turns them all a bit, but 

 the little weak ones, the violet, are turned much 

 more than the strong red, and all the intermediate 

 tints are turned in proportion to their speed of step 

 or their shortness or weakness. All three ideas are 

 important. Then, after passing the impulse of the 

 prism, again they go straight forward. If the crowd 

 be a very narrow one it corresponds with a narrow 

 slit. At once it begins to sort itself into its 

 constituent colours. The crowd broadens as it 

 travels ; after a time instead of a mixed crowd its 

 front is a long orderly line of individuals, a rainbow- 

 tinted streak, a spectrum, with the short violet 

 waves running at one end, and the tall red stalking 

 along at the other end. Supposing the individuals 

 of a certain height were truants, there would be a 

 gap, as when the electric arc shines through a 

 sodium flame. Supposing there be many truants of 

 different wave-lengths then there will be many gaps, 

 and we get the analogy of the solar spectrum. 



Heat and Chemical Action. 



White solar light is even more complex than the 

 coloured crowd : there are taller fellows than the 

 longest red and shorter than the shortest violet ; 

 they are in invisible grey, and the eye cannot see 

 them. The long waves are strong and the short 

 waves tricky. The big slow waves act as robber 

 brigands ; they can knock a whole molecule and 

 start it flying. The little waves are weak thieves 

 that cannot deal with a whole molecule, but can 

 steal his atoms. 



The invisible ultra-violet waves are better pick- 



pockets than the strong invisible infra-red. In the 

 solar spectrum the greatest energy is in the heat- 

 waves at and beyond the red end. The chemical or 

 actinic power of the spectrum is near and beyond 

 the violet end. The long waves are heating because 

 they move the whole molecules ; the short waves 

 are photographic because photography depends on 

 loosening and breaking up the molecule. The chloride 

 of silver is pulled to pieces and the silver deposited 

 by the quick vibrations of light that are not power- 

 ful enough to do much in the way of moving the 

 whole molecule and so heating the compound. The 

 height of the parts of the three curved lines in 

 Figure 520 gives the value in the entire solar spectrum 

 of its three properties ; that is, its energy or heat, 

 its light as it affects the eyes, and its actinism or 

 power to pull molecules to pieces. The power to 

 pull to pieces is also feeble energy, so energy is in 

 all parts of the spectrum ; but generally the higher 

 the temperature the greater is the ratio of light to 

 heat and the actinic power to the total energy. 

 Generally, also, the higher the temperature, the 

 greater the number and complexity of overtones the 

 atoms produce as they strike one another. Heat 

 being molecular indiscriminate motion, the velocity 

 of molecules increases with the temperature. 

 If a platinum wire be heated by an electric 

 current and used instead of a slit, and looked 

 at with a prism fixed over a hole in a large card with 

 the parallel edges of the prism held parallel to the 

 platinum wire, a good spectrum of the hot wire will 

 be seen. Of course, one does not look directly 

 towards the wire, but in the direction the rays are 

 bent by the prism. When the current is such that 

 the wire is just visible a red mass of light is seen. 

 As the resistance is lessened and the current 

 increases, the wire brightens, and as it does so the 

 red glow spreads to orange, then yellow is added, 

 and other successive colours of the rainbow show 

 themselves. When the platinum is near its fusing 

 point, a magnificently perfect spectrum of great 

 purity is produced. Thus we see the effect of 

 increased temperature in extending the spectrum 

 towards the violet ; that is, in increasing the speed of 

 movement of the molecules and consequently lessen- 

 ing the wave-lengths. This is the effect when we heat 

 a solid ; because its molecules cannot move freely, 

 it is like a mass of touching bells from which by 

 shaking we can only get a jangle of noise and no 

 music. The tightly fixed atoms of a solid have no 

 room for free motion, so they can only jangle and 

 give waves of every kind. 



When a bell rings alone it gives a musical note, 

 and when a vibrating atom is free in a rare gas it 

 gives colours ; often of many very pure tints. 



The colours in the spectrum of a star tell us its 

 story. The instrument we read the cipher message 

 with is the spectroscope, and the tale it tells us 

 depends on our understanding the laws of light and 

 widely correlated sciences. 



(To be continued.) 



