586 



NATURE 



[July 8, 1920 



so much in the faintness of the effect to be looked 

 for as in the avoidance of stray light which came 

 into competition with it. The essential thing is 

 to get a perfectly black background against which 

 the beam (viewed transversely) can be observed. 

 We cannot get this with a vessel like Tyndall's 

 tube just used. It is necessary to have what may 

 be called a black cave, and to view the beam as it 

 crosses in front of the mouth of the cave, the 

 latter forming the background. If the cave is 

 deep enough, there is no limit to the blackness 

 attainable. The great sensitiveness of the well- 

 rested eye, or the photographic plate, can then 

 be brought to bear, and the track of the beam 

 can be well seen, however carefully the dust is 

 removed. 



Some persons have been inclined to question 

 whether the dust is removed completely in these 

 experiments. As a matter of fact, this is not 

 where the difficulty lies at all. Dust so fine as to 

 be very difficult of filtration is an arm-chair con- 

 ception, not encountered in practical experiment- 

 ing. An enormous multiplication of the length 

 and tightness of the cotton-wool filter makes no 

 difference at all, a filter of modest dimensions 

 doing all there is to do. 



The dust particles which are originally present 

 in the air, near the ground or in a room, are 

 large, being in some cases individuallv visible to 

 the naked eye ; thus they do not fulfil the condition 

 for scattering a preponderance of blue light. The 

 molecules of air are, of course, amply small 

 enough, and the band of light seen stretching 

 across the mouth of the dark cave is, to my eyes 

 at least, of a full blue colour. In exhibiting the 

 effect to individual friends (and unfortunately it is 

 not bright enough to be shown to an audience), I 

 have been surprised and somewhat disconcerted 

 to find that they do not all see it blue as I do, but 

 some, for example, describe it as lavender. This 

 is undoubtedly due to a peculiarity of colour-vision 

 where faint lights are concerned. The ultimate 

 test is the spectroscope. Photographs of the 

 scattered light taken with this instrument clearly 

 show that the maximum of intensity is shifted 

 towards the blue, as compared with the original 

 exciting light. 



Polarisation of Light Scattered by Pure Air. 



A very important point to examine in connection 

 with the scattered light is its state of polarisation. 

 Visual examination with a Nicol's prism soon 

 showed that the polarisation was very nearly com- 

 plete. For closer examination I had recourse to 

 photography. It may perhaps be thought an 

 easier and more effective plan to look at a pheno- 

 menon than to photograph it, and no doubt it is 

 so in many cases : not, however, where the light 

 is very faint, but admits of long exposure. It 

 has long been recognised that photographs of the 

 nebulae will show much more than can be detected 

 visually by the keenest and most discriminating 

 eye. In this work on the scattering of light, I 

 have found it positively less trouble to take a 

 photograph than to make a visual observation, 

 NO. 2645, VOL. 105] 



even when the latter was feasible. The time 

 required to rest the eye in darkness and the effort 

 of attention required in observing a faint effect 

 cost the experimenter more than the exposure 

 and development of a plate. 



When the scattered beam in pure air is photo- 

 graphed, with a double image prism of Iceland 

 spar mounted over the photographic lens, it is 

 found that the polarisation is nearly complete, 

 but not absolutely so. However carefully the 

 instrumental adjustments are made and the 

 air filtered, I have found that there is a 

 slight residual polarisation indicating vibra- 

 tions parallel to the direction of the original beam. 

 The intensity of this residual polarisation, in what 

 may be called for convenience the wrong direction, 

 is about 4 per cent, of the whole. Now, as the 

 theory shows, there are two causes to which 

 failure of complete polarisation may be attributed. 

 One, which we may dismiss in this case, is that 

 the particles are not small enough. Another is 

 that they are not spherical — that is to say, 

 it is not a matter of indifference which way they 

 are presented to the primary beam. The latter 

 alternative may be illustrated by considering an 

 extreme case — namely, what we may call a needle- 

 like molecule, capable of vibrating only in one 

 direction fixed within it. Evidently such a mole- 

 cule when obliquely situated will have a com- 

 ponent vibration parallel to the direction of the 

 incident light. 



From the experimental fact that there is such a 

 component we may infer that the molecules of 

 air are not in the optical sense spherical. Experi- 

 ments on various gases have shown a character- 

 istic departure from complete polarisation, differ- 

 ent for each gas. Much effort has been spent on 

 determining the exact amount for each, and it is 

 hoped that the numbers obtained will form valu- 

 able material in the future for investigating the 

 structure of atoms and molecules. 



Polarisation of the Night Sky. 



We have seen that the polarisation of the day- 

 light sky is one of the most conclusive proofs that 

 its light is due to scattering by small particles. 

 What of the sky at night? Some of you will 

 perhaps be inclined to reply that the sky at night 

 is dark, and that the question whether its light is 

 polarised does not arise. It is, however, by no 

 means the case that the sky on a clear night is 

 absolutely dark, as anyone may readily prove by 

 holding his hand with outstretched fingers against 

 the sky. The fingers will appear dark against the 

 sky as a luminous background. 



The light is no doubt very faint, but I thought 

 it would be practicable to test whether it was 

 appreciably polarised or not. For this purpose 

 what is called a Savart polariscope was used. 

 Time will not allow us to consider the rather 

 complex theory of this apparatus ; it must suffice 

 to say that if the light which falls upon it contains 

 even a small part which is polarised, bands alter- 

 nately bright and dark are produced, which 

 further show colour due to the composite nature 



