402 



NATURE 



[November 24, 192 1 



follow out the idea in 1896, as did Adam Sedgwick in 

 1899. If authority be necessary, here is authority in 

 plenty. I also tried in my very humble way, beginning 

 as long ago as 1906. 1 daresay each man in turn 

 thought he was propounding something new. Dr. 

 Cunningham perceives, I hope, that Prof. Goodrich 

 is spared the disgrace of being my pupil, and that 

 even the most self-respecting biologist may, in this 

 instance, follow the truth without qualms of con- 

 science. Dr. Bather knows with what reception I 

 met. I was told that I was doing harm, that bio- 

 logists could manage their affairs quite well without 

 my help, and so forth. Then the worm turned. So 

 far as I am able to judge. Dr. Bather objects 

 to my letters because they are tediously long and 

 because they are impudent. Certainly they are 

 long, and doubtless they are tedious. But I could 

 state, or assume, in half a dozen words a fallacy which 

 Dr. Bather could not refute in less than half a dozen 

 columns. Moreover, as Dr. Bather courteously indi- 

 cates, it has been holiday time, during which one 

 does unusual things ; therefore I have used his letters 

 —with all reverence, as a parson might — as texts 

 whereon to hang admonitory discourses. Certainly 

 these letters have been impudent — most impudent. 

 But here, again, we have the trodden worm. 



Dr. Bather thinks I ought not to discuss variations 

 unless I first account for them, which is like saying 

 I ought not to eat my dinner unless I first 

 cook it. Must I not accept the given fact? I 

 am at once accused of being tediously long and 

 not long .enough. What is a poor man to do? 

 Besides, I have tried elsewhere (" The Laws of 

 Heredity," chap. 5) to do this very thing. 

 Primarily variations can arise only in two ways. 

 Either they are impressed on the germ-plasm by its 

 environment, or thev occur because the germ-plasm 

 is a living, growing, changing thing which, like other 

 living things, tends to revert to the normal from 

 impressed change, especially injury. There exists 

 ample crucial evidence to enable us to reach a 

 decision, but much of it lies, outside the high roads 

 followed by biologists, in the realms of disease and 

 bacteriology. 



Dr. Cunningham's letter (Nature, November 17, 

 p. 368) is addressed especially to Prof. Goodrich, who 

 may deal with it if he desires ; but one passage refers 

 to my particular hobby. Lamarck called the changes 

 which result from use "acquired "; but, thinking only 

 of trifling changes which occur at the end of the 

 development, he did not realise that the growth of 

 the higher animals, especially man, is due mainly to 

 that functional activity which begins to act imme- 

 diately after birth. His successors employed the 

 word " acquired " as indicating anv character which 

 develops under anv very glaring influence. Now Dr. 

 Cunningham defines an acquired character as a 

 "change " (from the person's antecedent self, from 

 the parent, from the race — which?) due to "environ- 

 ment or modification." In that case the English 

 language is not "acquired," but is "innate" in an 

 Englishman. If learned by a Frenchman, it is 

 acquired. Heaven knows what it is if learned in 

 Jersey. He accuses Prof. Goodrich and me of a 

 "misuse of words" and of obscuring "a perfectly 

 clear distinction"! The italics are mine ! 



G. Archdall Reid. 



9 -Victoria Road, Southsea, November 19. 



The Softening of Secondary X-rays. 



Dr. a. H. Compton in a letter on this subject in 

 Nature of November 17, p. 366, described an experi- 

 ment in which he reflected the Ka rays from a molyb- 

 NO. 2717, VOL. 108] 



denum Coolidge tube on to a slab of paraffin, arid 

 measured the absorption coefficient of the secondary 

 scattered rays at different angles with respect to the 

 direction of the primary beam. The absorption co- 

 efficient of the secondary rays was found to be 29 per 

 cent, gr'eater than that" of the primary Ko beam at 

 ^ = 90°, and 6 i>er cent, greater at (9 = 20". This soften- 

 ing of the rays on being scattered was still more 

 pronounced when the K lines of tungsten were used. 



Dr. Compton referred to this work as a repetition 

 of measurements which I had previously reported 

 {Phil. Mag., September, 192 1), in which no such in- 

 crease in absorption after scattering was observed, 

 and he attributed my negative result to an unfavour- 

 able choice of wave-length and angle. Apparently he 

 did not understand the purpose of my experiment" It 

 was to settle a question regarding the interpretation 

 of energy measurements made with the Bragg spectro- 

 meter. We were not sure that the atom in a scat- 

 tering substance does not always absorb energv from 

 the incident rays and re-emit this energy in a manner 

 characteristic of the atom and independent of 6. My 

 problem was to find out if such an effect need be 

 considered in ordinary spectrometer measurements. 

 The wave theory of scattering predicts a certain 

 amount of softening due to the finite size of the atom 

 and to a sort of Doppler effect, but not nearly the 

 observed amount, especially at large angles. 



As Dr. Compton suggests, there is probably an addi- 

 tional somewhat softer radiation due to collisions of 

 electrons released within the scattering substance by 

 the primary rays. Such a "fluorescent" radiation 

 should diminish with 9, as observed. The softening 

 due to the finite size of the atom should also, in 

 general, diminish with 6 and be negligible in the 

 characteristic radiation, which is believed to consist 

 of relatively sustained wave-trains. Softening due to 

 these recognised causes can thus be minimised by 

 using the sustained characteristic rays, large wave- 

 lengths, and 9 as small as possible. I chose these 

 conditions, which were unfavourable to the Compton 

 effect, because I wanted to eliminate it so far as 

 possible. The negative result simplv indicates that 

 with light atoms the indirect unpolarised radiation 

 sought is not great enough to require consideration 

 in ordinary crystal measurements. S. J- Plimpton. 



Worcester Polytechnic Institute, Worcester, 

 ^Massachusetts, November 8. 



The Molecular Scattering of Light in Liquids and Solids. 



As was pointed out by the late Lord Rayleigh, thei 

 basis of his theory of the blue sky, namely, that the ' 

 molecules scatter the incident energy independently of 

 each other's presence, is only true for gases in con-, 

 sequence of the freedom of movement the molecules] 

 possess in this state of matter. In connection with 1 

 the problem of the colour of the sea and of deep! 

 waters generally it is necessary to know the scatter-| 

 ing power of ordinary liquids, such as water, and I' 

 find this can be very simply accomplished by applica-i 

 tion of the theory of local fluctuations of density aris- 

 ing from molecular movement, originated by Einstein 

 and Smoluchowski and utilised by the latter to eluci- 

 date the phenomena occurring near the critical state. 

 The general formula for the scattering power of a 

 fluid is 



TTjRTS/ 2 



18 N\*V 



where /3 is the compressibility of the substance, fx its 

 refractive index, R, T, N being the usual constants of 

 the kinetic theory. The scattering power of water 

 comes out from this formula as about 160 times that 



'-iy(M-+2 



