Junk. 1911. 



KNOWLEDGE. 



239 



line spectrum has been termed the '" flash " spectrum. 

 Before bein? able to give the true explanation of the flash 

 spectrum, it is necessary to explain what is meant by 

 dispersion. 



If white light be passed through a glass prism, it is not only 

 bent and deviated by an amount proportional to the 

 "refractive index" of the glass, but it is also split up into a 

 spectrum, the red light being less deviated than the violet; 

 this is termed the "dispersion" of the light. Now light is 

 refracted more by a dense glass prism than by a light crown 

 glass prism of the same angle, but it does not necessarily 

 follow that the dispersion is proportional to the " refractive " 

 power {i.e., to the refractive index! of the glass. It is this 

 that makes it possible to construct achromatic prisms and 

 lenses : the dispersion of one prism can be counteracted by 

 that of a dense flint glass prism though the deviation or 

 refraction of the beam of light is still obtained. Newton 

 arrived at the conclusion that dispersion was proportional to 

 refraction; this was erroneous. If the refractive indices be 

 plotted against the wave-lengths of the light passed through 

 various substances, it is not always found that the substances 

 of high mean refractive index give great dispersive powers, or 

 steep curves, when plotted as mentioned. 



Kundt found in examining the absorption spectra and 

 dispersion curves of many dyes, that such highly coloured 

 substances which show strong absorption bands, give what 

 has been termed " anomalous dispersion " in the neighbour- 

 hood of these bands. Instead of increasing as the wave-length 

 decreases, the refractive index increases very rapidly on the 

 red side of the absorption band and decreases towards the 

 blue side of the band. " Normal " dispersion is merely a 

 p.irticular case of the general phenomena of dispersion ; the 

 band near which the dispersion is " anomalous " lying, for 

 transparent colourless substances, in the ultra violet invisible 

 region of the spectrum. 



Returning now to the sun and considering its " flash " 

 spectrum, it follows that the refractive index in the neighbour- 

 hood of an absorption band, due to some vapour or other, will 

 increase very rapidly : it will be very small except close to the 

 absorption band, hence light nearly corresponding in wave- 

 length to the absorption band will alone be sorted out and 

 bent sufficiently for it to reach the earth, when the moon has 

 blocked out all direct light from the photosphere. This 

 beautiful explanation is due to W. H. Juhus, but Professor 

 Wood has been able to reproduce the phenomenon experi- 

 mentally, thus confirming the correctness of the theory. 



It will be well first to describe Professor Wood's method of 

 illustrating the anomalous dispersion of sodium vapour. A 

 long glass tube with pieces of sodium strewn on the bottom is 

 fitted w'ith plate glass ends and exhausted. It is then heated 

 by carefully adjusted Bunsen burners. The upper surface of 

 the tube being cool, the density of the vapour decreases as its 

 distance from the heated sodium. It then forms what is 

 equivalent to a prism of sodium vapour. Light from an arc 

 lamp passed through a slit and then through the sodium prism 

 is absorbed by the vapour which, by the way, when dense, has 

 a blue violet colour ; particularly absorbed are those wave- 

 lengths corresponding to the yellow " D " lines. There are 

 also absorption bands in the green and red. Now, in the 

 neighbourhood of the " D " absorption bands, the vapour will 

 have an abnormally high refractive index for w-a\'es slightly 

 longer than those which it absorbs and an abnormally low 

 refractive index for waves slightly shorter than those which it 

 absorbs. Thus yellow light passing through the sodium 

 vapour prism will be very highly dispersed, the wave-lengths 

 on either side of the absorption bands being most widely 

 separated. If the light be now observed in a spectroscope with 

 the refracting edges of the prisms at right angles to the 

 edge of the sodium prism, the light in the neighbourhood of 

 the " D " absorption lines will curve up on the one side of the 

 band and down on the other side owing to the great dispersion 

 of the sodium vapour for light of wave-lengths in this 

 neighbourhood. Professor W'ood was able to project the 

 artificial " flash " spectrum of sodium on the screen. Light 

 from an arc is projected through a narrow horizontal slit 

 along the lower edge of a metal plate heated by a Bunsen fed 



with sodium which colours the flame \-ellow. The light on 

 lea\ing the plate is almost completely screened off, passed 

 through a prism and projected on the screen. The metal 

 plate lowers the temperature of the flame and the sodium 

 vapour no longer emits light in its neighbourhood. The light 

 passing through this layer of sodium vapour is anomalously 

 dispersed in the yellow region. The rays on eii'.ier side of the 

 absorption bands are refracted differently, the vine ray being 

 bent upw-ards and the other down. The screen being adjusted 

 to cut oft' the direct light from the arc, the yellow light bent 

 upwards suddenly "flashes" out on the screen. This then 

 illustrates what is occurring in the sun's atmosphere. 



The second lecture dealt with the emission of light by vapours. 

 X'apours can be made to emit light by the passage of electricity 

 through them when the electrons in the ions are greatly 

 disturbed and set in vibration. The disturbance in this case 

 is eftected by many causes ; it is better then to look for a less 

 complicated method of setting the electrons in vibration in 

 order to discover something about the structure of the atom. 

 Those vapours which absorb light and hence have a definite 

 colour can be made to emit light by heating them. Professor 

 Wood showed how- iodine dropped into a quartz bulb heated 

 to a high temperature, is \aporised and glows^the vapour 

 appearing " red-hot." White light passed obliquely 

 through a steel tube in which sodium is vaporised 

 causes the sodium to fluoresce or emit light of 

 a green colour by stimulation. In the same way light 

 passed through a bulb containing iodine vapour at a very 

 low pressure causes the vapour to fluoresce with an olive green 

 hue. The presence of a small quantity of helium changes 

 this hue to a reddish tint, while other gases destroy the 

 fluorescence in proportion partly to their molecular weight 

 and partly to their electro-negative character ; a very 

 small quantity of chlorine destroys the fluorescence altogether. 

 Mercury vapour will fluoresce at much higher pressure ; 

 Professor Wood illustr.ated this point by projecting light from 

 a magnesium spark on a quartz bulb in which mercurv was 

 boiled : the mercury vapour, when all the air had been expelled 

 from the flask, lit up with a blue colour. Iodine when heated 

 emits light of the same wave-length as it absorbs : the electrons 

 in the molecule are set in vibration by the process of heating 

 the vapour and give out particular wave lengths of light, just 

 in the same way as these waves are absorbed on passing 

 through the vapour, their energy being absorbed by setting in 

 motion those electrons which vibrate with the same period of 

 vibration. White light passed through such a vapour, 

 provided the pressure is not great enough to prevent the 

 free vibration of the electrons by the collision of the mole- 

 cules, will give rise to fluorescence. If instead of employing 

 white light, light of a particular wave-length be employed, then 

 this light will aft'ect only certain electrons which vibrate with 

 the same period, and these in turn being grouped with only a 

 few other electrons in the molecule will set in vibration only 

 a few other electrons possessing different periods of vibration ; 

 only a few lines then appear in the spectrum of the light from 

 iodine vapour set resonating by the light from the mercury 

 arc. Professor Wood has studied these resonance spectra 

 most elaborately and amassed much interesting material from 

 which to obtain a glinipse of the atomic structure. 



The third lecture dealt with the eft'ect of magnetism on the 

 optical properties of vapour. Light from an arc lamp was 

 passed through a well-evacuated tube containing sodium 

 vapour placed across the poles of an electromagnet; the light 

 before it entered the tube was polarised by a Nicol prism and 

 only permitted to vibrate in one plane. On emerging from 

 the tube, the light was passed through a second Nicol prism, 

 so that the prism either permitted the polarized light to pass 

 or, by setting it at right angles to the plane of vibration of the 

 waves, it could prevent the light from reaching the screen. An 

 ordinary glass prism refracted the emerging light on to a 

 screen and formed a spectrum on the screen. The second 

 Nicol was so arranged that it almost extinguished the spectrum. 

 The magnetic field was switched on and immediately the sodium 

 yellow lines could be easily seen on the screen. In this experi- 

 ment the magnetic field rotates the plane of polarization 

 round through a right angle or some multiple thereof, thus 



