488 



NATURE 



[June i6, 192 i 



viscera are supported. With this sacralisation of the 

 tail there are numerous correlated changes in the 

 vertebras and muscles of the spine, in the musculature 

 of the body-wall and thorax, and in the shape and 

 arrangement of the viscera of the body-cavities. 



As will be seen from his letter, Prof. Wood Jones 

 is of opinion that each member of this orthograde 

 group of Primates — man, gorilla, chimpanzee, orang, 

 and gibbon — has acquired the sacralisation of 

 the tail independently of each other; in his opinion 

 we are dealing with remarkable resemblances 

 produced by convergence. On the other hand, it 

 seems to me a mt>re rational explanation to believe 

 that evolution is true, and that all the orthograde 

 Primates are the progeny of a common stock — the 

 primitive orthograde stock — and that we are there- 

 fore dealing with a common inheritance. Seeing that 

 all have a nervous system cast in a common mould, 

 with vascular, alimentary, muscular, and bony systems 

 which differ only in detail, we do much less violence to 

 what we know of the laws of evolution by adopting 

 my explanation than if we accept that offered by Prof. 

 Wood Jones. In no other Primate, save the five 

 mentioned above, has the tail undergone sacralisation. 

 The guinea-pig, the Barbary ape, and Cynopithecus 

 have no bearing on the point in question ; their tails 

 have not undergone sacralisation. To compare the 

 posture and method of progression of the kangaroo 

 to man or an anthropoid ape is of the nature of 

 burlesque. 



In my original paper on vestigial tails I made special 

 allusion to Tarsius because Prof. Wood Jones has 

 misled public opinion as to the structural relationship 

 that exists between anthropoid apes and man. He 

 holds, on what 1 consider a flimsv basis, that man 

 has been evolved from a Tarsius-like ancestor, and 

 that between this ancestor and man there must be a 

 series of undiscovered links. Tarsius has a particu- 

 larlv long tail ; in no sense can its posture or method 

 of progression be said to be like that of the ortho- 

 grade Primates. In the manner in which its tail- 

 muscles are arranged Tarsius resembles pronograde or 

 dog-like aoes. It has no claim to be called humanoid, 

 whereas in this, as in a thousand other structural 

 characters, the anthropoid apes can claim, not a 

 resemblance, but an identitv. Arthur Keith. 



The Stationary H- and K-lmes of Calcium in Stellar 

 Atmospheres. 



It has been noticed by many observers that the 

 space surrounding early B-classes of stars {^-g- 

 S Orionis) often show absorption of H- and K-lines 

 of calcium, which do not share in the Doppler dis- 

 placements of the other absorption-lines of the stellar 

 spectra. This suggests that these stars are enveloped 

 in an atmosphere of calcium vapour which does not 

 partake in the orbital motion of the stars (Nature, 

 April 21, p. 247). 



There is, of course, naturally a diflficultv in realising 

 why calcium, alone of all elements, should be found 

 to occur in the attenuated atmospheres surrounding a 

 stellar system. Ver^' closely connected with this 

 phenomenon is the observational fact that in the flash- 

 spectrum of the sun the long"est arcs are those corre- 

 sponding to calcium H- and K-lines, indicating that 

 in the sun also the outermost layers (according to 

 Mitchell, 14,000 km. above the solar disc) are com- 

 fx>sed of calcium. Hydrogen, the lightest of elements, 

 which we should expect to occur in the highest 

 layers, disappears at a much lower level (8000 km., 

 according to Mitchell). 



The problem is naturally a complicated one, but 



NO. 2694, VOL. 107] 



I think that a way to solution is afforded by the 

 theories of selective radiation-pressure and of the tem- 

 perature-ionisation of gases advanced by me in 

 the following papers : — " On Radiation Pressure and 

 the Quantum Theory " {Astrophysical Journal, Septem- 

 ber, 1919); "On Selective Radiation Pressure, etc." 

 (Journ. Coll. of Science, Calcutta, 1920) ; " lonisation 

 in the Solar Chromosphere, etc." (Phil. Mag., vol. 

 xl., 1920); and "On a Physical Theory of Stellar 

 Spectra " (Proc. Roy. Soc. Lond., May, 192 1). 



According to these papers, the H- and K-lines are 

 the resonance-lines of Ca+, i.e. of a calcium-atom 

 which has lost one electron. The resonance-line of 

 neutral calcium is the ^-line, A = 4227. In the Fraun- 

 hofer spectrum we get H, K, and g, showing that 

 in the solar photosphere calcium is largely ionised 

 owing to the high temperature prevailing there. At 

 higher levels, owing to diminution in concentration, 

 the ionisation becomes complete, so that the j^-line 

 disappears entirely, leaving onlv the H- and K-lines. 



The sun is a dwarf star of the Go class, corre- 

 sponding to a surface temperature of 7000-7500° K. 

 When we consider the spectra of the still hotter 

 stars, classes F, A, and B, we find that the ^-line 

 becomes fainter and fainter, until it disappears alto- 

 gether from the B8A class. In the still hotter stars 

 we have only the H- and K-lines, showing that they 

 do not contain neutral calcium at all, but only ionised 

 calcium. 



This explains the varying behaviour of the ^-line 

 and of the H- and K-lines, but we have still to deter- 

 mine the force which drives Ca+ to the outermost 

 layers. It is natural to conclude that the forces which 

 are responsible for driving calcium absorbing H and 

 K to the greatest height in the solar atmosphere 

 are also responsible, in the case of stars having a 

 larger surface temperature, for driving calcium to 

 the surrounding parts of space. Now what can this 

 force be. and why should this show a preference for 

 calcium ? 



In the case of the sun I have attempted to show 

 that this force is furnished by the pressure of radiant 

 energy from the solar disc acting in a selective way 

 upon the Ca+-atoms. The term " selective " is most 

 important here, and requires an explanation. Radia- 

 tion-pressure is due to absorption, and therefore, in 

 the case of a g^as illuminated by white light, only 

 those pulses which the gaseous atom is capable of 

 most frequently absorbing are effective in producing 

 pressure. A gas can usually absorb lines of the 

 principal series alone, but the lines of the subordinate 

 series are absorbed only in exceptional circumstances, 

 and even then to a much smaller extent ; so that the 

 maximum lifting effect of radiation-pressure is to be 

 expected only in the case of atoms absorbing the 

 resonance-lines. (For more detailed argurnents see 

 the papers above-mentioned.) In addition to this, the 

 lifting force would depend on the intensity of the 

 region corresponding to the absorbed lines in the 

 spectrum of the continuous background of white 

 light, and on the solid angle subtended at the atom 

 bv this background. 



In the case of the sun the surface temperature is 

 7300-7500° K (Biscoe, Astrophvsical Journal, vol. xlvi., 

 P- 355)' so that, according to Wien's law, \^T = b, 

 the maximum of emission lies at A = 3920 A.U., very 

 close to the H- and K-lines of Ca+. Also these lines 

 are the resonance-lines of Ca+, so that we have here 

 the maximum effect of selective radiation-pressure. 

 The resonance-line of hydrogen is at A=i2i6 A.U., 

 and therefore the effect of radiation-pressure is 

 extremely small. 



It is not possible to say whether the lifting power 



