92 



NATURE 



[October 2, igig 



rather rapid and bewildering change consequent 

 on advances in positive knowledge. Absorption 

 spectra, the properties of colloids, ionisation and 

 the nature of ions, the nature and source of 

 osmotic pressure, and the relations of isotopes are 

 all subjects of supreme interest, many of which 

 have assumed a totally new form, or have even 

 been recognised only within the last twenty years. 

 The chemical student of the future will need to 

 be a fairly good mathematician if he hopes to 

 follow all that is going on in these several direc- 

 tions. Fortunately there are other large fields of 

 work still open in which this is not an essential 

 condition and where great successes continue to 

 be scored, especially in constitutional and syn- 

 thetic organic chemistry and its applications to 

 problems in physiology, animal and vegetable. 

 These are all dealt with under appropriate heads 

 in this volume of reports. 



Heredity. By Prof. J. Arthur Thomson. Third 

 edition. (The Progressive Science Series.) 

 Pp. xvi + 627. (London: John Murray, 1919.) 

 Price 15s. net. 

 The first edition of Prof. Thomson's "Heredity," 

 which appeared in 1908, was reviewed at some 

 length in Nature (vol. Ixxviii., pp. 361-63). The 

 book quickly became established as an introduc- 

 tion — at once trustworthy, impartial, and com- 

 prehensive — to the many problems that are pre- 

 sented to students of inheritance, and a second 

 edition with some additions and revisions was 

 published in 191 2. The third edition is now before 

 us, and the author has taken the opportunity of 

 directing the reader's attention to some of the 

 important advances that have been made by in- 

 vestigators during the last seven years. The size 

 of the book has not been increased from the second 

 edition, so that room for additions has been found 

 by condensing the type-setting on certain pages ; 

 this involves a brevity of treatment disappointing 

 to those who would have valued Prof. Thomson's 

 judicious criticism of several recent theories. For 

 example, the studies by T. H. Morgan and his 

 fellow-workers on the inheritance of linked factors 

 in the fruit-flies (Drosophila), and W. E. Castle's 

 work on the relation between heredity and selec- 

 tion in hooded rats, are barely mentioned. 



A short list of some important books and papers 

 of the last few years has been added to the biblio- 

 graphy, but the subject and general indexes appear 

 to have escaped a revision which would have 

 greatly increased their value. The paragraph on 

 " Militarism " in the concluding chapter has been 

 rewritten in the light of the experiences of the 

 last five years, and the author emphasises Dr. 

 Chalmers Mitchell's contention that "the struggle 

 for existence as propounded by Charles Darwin 

 and as it can be followed in Ndture has no resem- 

 blance with human warfare." Again, as one turns 

 the pages of Prof. Thomson's familiar volume, 

 one realises how the study of biology, wisely 

 applied, may become an aid rather than a rival to 

 that of "the humanities." G. H. C. 



NO. 2605, VOL'. 104] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can be undertake to return, or to correspond -with 

 the writers of, rejected manuscripts intended for 

 this or any other part of NATfRK. .Vo notice is 

 taken of anonymous cointtiunications.'] 



A Photoelectric Theory of Colour Vision, 



Referring to his letter under the above heading on 

 p. 74 of Natlre of September 25, I perceive that 

 Prof. Joly has had ideas similar to mine about an 

 electric stimulation of the terminals of the optic nerve 

 through bombardment of corpuscles flung ofl under 

 the stimulus of ordinary light. 



My argument is strengthened by reflecting that this 

 utilisation of atomic energy, emitted in quanta under 

 the stimulation of accumulated almost infinitesimal 

 \ibrations of the right frequency, can account for the 

 extreme sensitiveness of the eye and of the sensitive 

 pigments known very low down in the scak- of animal 

 life. The great variation of brightness permissible, 

 between wide limits, without much differential physio- 

 logical result is also natural on this view ; so is the 

 fatigue of colour-sensation by temporary exhaustion 

 of a specific potentially radio-active material, until 

 renewed by living tissue. 



I should suppose that on this trigger-like basis the 

 eve can form very little estimate of absolute bright- 

 ness inside the limits above spoken of, though 

 the ear, having no explosive mechanism, might be 

 able to form a scale of loudness. In the main, photo- 

 metric observations must be comparative. 



A pathological condition of the retina, when flashes 

 are perceived without objective stimulus, may be 

 accounted for by overinstability of material and con- 

 sequent spontaneous emission of corpuscles. 



The experiments which Prof. Joly began to try 

 seem to have been just in the general direction which 

 I wished to encourage some young physiological 

 phvsicist to pursue, only he must be prepared to 

 design or adjust his electrical detecting instrument 

 for extreme sensitiveness. A frog's nerve-rnuscle 

 preparation could scarcely be responsive without 

 something analogous to rods and cones, or some- 

 thing like an electric organ, and without access to 

 unsheathed terminals. If a mechanical electroscope 

 is emploved it must have minute capacity; a silvered 

 quartz filament, with a minimum of attachments, in 

 the field of a microscope mav be suggested. 



Oliver I. odor. 



Reversed Pleochroic Haloes. 



In a paper on "The Genesis of Pleochroic Haloes" 

 (Phil. Trans. R.S., vol. ccxvii.) by J. Joly, a theory 

 is advanced accounting for certain structural features 

 of the halo on the assumption that reversal of the 

 halo-image is possible, and mav take place under 

 conditions defined in the paper. In support of_ this 

 a drawing of a halo is given in which an evident 

 inversion or change from positive to negative has 

 occurred, the inner' region being light, the outer dark. 



Recondy, in examining the brown mica extracted 

 from a granite, we have found quite a large number 

 of these negative haloes. -Ml internal features are 

 gone, solarised out of existence; the wide outer band 

 alone remains. Thev resemble negatives of a much 

 exposed halo. Their dimensions shows that the; 

 possess uranium -charged nuclei. When the nucleus 

 is very minute there is no sign of reversal; the halo 



is normal. , ■ u- 



It is possible that the frequency of reversal m this 



