NATURE 



[May 5, 1898 



and alumina and their cryslallographic allies." But the 

 explorer should clearly understand that, notwithstanding 

 occasional accidents, the most trustworthy results in the 

 search for minerals will, in the long run, be reached by 

 that man who brings to bear upon his work the widest 

 range of scientific knowledge. 



The Process of Creation Discovered ; or, the Self-evolution 

 of the Earth and Universe by Natural Causes. By 

 James Dunbar. Pp. viii 4- 290. (London : Watts and 

 Co., 1898.) 

 To review this book would be to give prominence to a 

 volume every page of which exemplifies the dangerous 

 character of a little knowledge. We will merely remark 

 that the author finds himself at variance with very many 

 physical facts and theories, disbelieves the results of 

 spectroscopic analysis applied to celestial bodies, and 

 regards the solar photosphere as a deep ocean of water. 

 According to his theory of inorganic evolution, " the 

 only elements employed or necessary in the formation 

 of the sun, solar system, and universe are those com- 

 posing atmospheric air and water.'' Students of science 

 may be left to form their own opinion upon a book 

 containing an assertion of this kind. 



Domestic Science Readers. Book vii. By Vincent T. 

 Murehe. Pp. 298. (London : Macmillan and Co., 

 1898.) 



The subject of domestic economy is taught in the 

 various standards of our elementary schools ; and this 

 book is adapted to supply girls in the highest standards 

 with the information which the Education Department 

 expects them to possess. The laws of health, infant 

 management, common ailments and their remedies, 

 common accidents, infectious diseases, and management 

 of the sick-room are the subjects dealt with in the six 

 parts of the book, and they are treated in a very clear 

 and instructive manner. Mr. Murch^ knows how to 

 interest the young readers for whom he writes, and this 

 little school book will doubtless be as successful as the 

 others of which he is the author. Moreover, the pupils 

 who read the book will receive a large amount of sensible 

 advice which will give them a sound understanding of 

 the laws of health, and thus be of service to them and to 

 future generations. 



A Course in Mechanical Drawing. By John S. Reid. 



Pp.128. (New York : John Wiley and Sons. London: 



Chapman and Hall, 1898.) 

 Teachers of the elements of mechanical drawing to 

 students in marine, electrical, railway, and mechanical 

 engineering will find that this book contains a concise 

 statement of the essential principles of the subject. In 

 the five chapters, the author deals with drawing instru- 

 ments, geometrical drawing, or the use of the instruments, 

 conventional methods of drawing used by draughtsmen, 

 lettering and figuring, and orthographic projection. The 

 author is instructor in mechanical drawing and design- 

 ing in Sibley College, Cornell University, and his ex- 

 perience has enabled him to produce a useful work. 



Flotver Favourites., their Legends., Symbolism and 

 Sii^nificance. By Lizzie Deas. Pp. viii -1-229. (London: 

 George Allen, ]898.) 

 Many pretty stories concetning common flowers have 

 been collected from folk-lore and classic myths by the 

 author, and are presented here in an attractive setting. 

 The nursery traditions and love legends referring to 

 flowers and flower-names are numerous and interesting 

 enough, but very little attention is devoted to the subject 

 of " plants and flowers in their widest relationships " 

 referred to in the preface. 



NO. 1488, VOL. 58] 



LETTER TO THE EDITOR. 



[ 77^1? Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he laidertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of NATURE, 

 No notice is taken of anonymous coviinuttications.^ 



Rontgen Rays and Ordinary Light. 



I QUITE agree with the physical principles in Lord Rayleigh's 

 article on "Rontgen Rays and Ordinary Light " in Nature of 

 April 28, and think that the difference between us is one of 

 terminology. I am accustomed to restrict the word wave to 

 disturbances in which the harmonic character is well developed, 

 and not to use it in physics in the sense in which it is used in 

 the phrase "a wave of enthusiasm." It would never have 

 occurred to me to speak of a disturbance localised in a thin 

 shell as a wave of short wave-length. I should speak of it as a 

 pulse, and though such pulses can of course be resolved by 

 Fourier's theorem into trains of waves, yet it seems to me that 

 when a simple pulse is so resolved (except for some special 

 purpose), there is a loss of clearness both in expression and con- 

 ception analogous to that which would occur if we regarded a 

 straight line as an aggregate of harmonic curves. 



The term pulse has the advantage that it suggests the funda- 

 mental property of the Ronlgen rays, that their action on matter 

 in their path is an impulsive action, i.e. that the time constant 

 of the disturbance (the time taken by the pulse to pass over a 

 point) is small compared with the time constant of the system 

 ill their path (the time of vibration of the molecules). 



I am not aware that I have ever regarded these pulses as 

 possessing any physical property which would be inconsistent 

 with the physical properties of the constituents into which they 

 can be resolved by Fourier's theorem. Personally I should 

 expect that if a train of waves of wave-length A were refracted, 

 a pulse of thickness A would be refracted too, and if the thick- 

 ness of these pulses were of the order of the wave-length of 

 ordinary' light, that the Rontgen rays would be like ordinary light. 



I believe the Rontgen rays to be pulses rather than waves of 

 small wave-length, not because I think the properties of the 

 latter would be different from those of Rontgen rays as far as 

 we know them, but because electromagnetic theory shows that 

 pulses, and not short waves, are produced by the impact of 

 kathode rays. J. J. Thomson. 



Cambridge, April 30. 



SLEEP, AND THE THEORIES OF ITS CAUSE. 



THE theory of the origin of sleep which has gained the 

 widest credence is the one that attributes it to 

 anaemia of the bram. It has been shown by Mosso, and 

 many others, that in men with defects of the cranial wall 

 the volume of the brain decreases during sleep. At the 

 same time, the volume of any limb increases as the peri- 

 pheral parts of the body become turgid with blood. In 

 dogs, the brain has been exposed, and the cortex 

 of that organ has been observed to become anaemic 

 during sleep. It is a matter of ordinary observation that 

 in infants, during sleep, the volume of the brain becomes 

 less, since the fontanelle is found to sink in. It has been 

 supposed, but without sufficient evidence to justify the 

 supposition, that this anitmia of the brain is the cause 

 and not the sequence of sleep. The idea behind this 

 supposition has been that, as the day draws to an 

 end, the circulatory mechanism becomes fatigued, 

 the vasomotor centre exhausted, the tone of the 

 blood vessels deficient, and the energy of the heart 

 diminished, and thus is the circulation to the cerebral 

 arteries lessened. By means of a simple and accurate 

 instrument (the Hill- Barnard sphygmometer), with which 

 the pressure in the arteries of man can be easily 

 reckoned, it has been recently determined that the 

 arterial pressure falls just as greatly during bodily rest as 

 during sleep. The ordinary pressure of the blood in the 

 arteries of young and healthy men averages 1 10-120 mm. 

 of mercury. In sleep, the pressure may sink to 95-100 



