March 8, 1888] 



NATURE 



437 



by friends in the course of last year. They bring out 

 indirectly all the qualities of Emin's character, and no 

 one can read them without being filled with admiration 

 for his sustained enthusiasm, his inexhaustible energy, 

 and his unaffected simplicity and modesty. He has 

 been too much occupied with official duty to devote as 

 much time as he would have liked to scientific investiga- 

 tion ; but he is an ardent student of zoology, botany, and 

 ethnography, and he says enough to show that we may 

 expect from him hereafter very important contributions 

 to our knowledge of all these subjects. So far as the 

 present volume is concerned, the most valuable of the 

 letters, from a scientific point of view, are those relating 

 to the various tribes whose habits and customs he depicts. 

 His descriptions are remarkably vivid, and are evidently 

 the result of much careful observation. His description 

 of the Wanyoro, for instance, is a model of what such a 

 piece of work ought to be. The writer omits no character- 

 istic that is like'y to be suggestive to anthropologists, 

 while he has taken care not to spoil the general effect of 

 his sketch by the intrusion of unnecessary details. Dr. 

 Felkin's introduction is written with perfect tact and 

 judgment, and Mrs. Felkin has done her work as a trans- 

 lator admirably. An excellent map has been prepared 

 by Mr. Ravenstein, who has also done good service by 

 marking the latitude and longitude of every place men- 

 tioned in the index and glossary. 



Colour. By A. H. Church, M.A. (London : Cassell 

 and Co., 1887.) 



In a work which has been limited to somewhat less than 

 200 pages, there has of necessity been a good deal omitted 

 which would have been found in a larger work. In the 

 part devoted to the production of the spectrum, the details 

 are almost absent in some particulars and perhaps are 

 rather too full in others. The subject of polarized light 

 is also dismissed too briefly. There are one or two state- 

 ments to which exception can be taken. The first is 

 where the author states (p. 44) that " calorescence may 

 be regarded as a variety of fluorescence." The intro- 

 duction of the term calorescence at all is a mistake ; but 

 it is a greater mistake to mix it up with what is a really 

 distinctive phenomenon. 



Another is at p. 78, where the author says, when 

 speaking of a person who is " red " colour-blind, that 

 ** the nerve fibrils which in the normal retina receive 

 the sensation of red are not, indeed, wanting, but transmit 

 to the brain the same sensation as that transmitted by 

 the second set of fibrils, the green." This doctrine is 

 rather against facts : the fibrils are either wanting or 

 else are paralyzed, as the total amount of light perceived 

 by the red colour-blind person in white light is less than 

 that perceived by the normal-eyed person. The sensa- 

 tions of the green and blue primary colours are on the 

 average equal in both, but the normal-eyed person has in 

 addition the red sensation. If the fibrils which in the 

 normal-eyed person respond to the red respond to the 

 green in the red colour-blind person, this would not be 

 the case. 



With these and one or two minor exceptions the work 

 is to be recommended for accuracy ; and the author may 

 claim to have accomplished what he states in his preface 

 he has endeavoured to do, viz. " to present and to explain 

 in a concise yet popular form many of the chief facts 

 connected with the origin, the phenomena, and the 

 employment of colour." 



Astronomy for Amateurs. By J. A. Westwood Oliver. 

 (London ; Longmans, Green, and Co., 1888.) 



This volume, to quote the preface, " is intended to afford 

 the amateur astronomer, possessed of limited instrumental 

 means, but yet anxious to devote his labours to the further- 

 ance of astronomical science, such hints and suggestions 

 as will help him to direct his efforts into the channels 



which experience has indicated as best fitted to his quali- 

 fications and equipment." Its pages are accordingly 

 entirely devoted to practical astronomy, theories of every 

 description being disregarded. The different branches of 

 the subject are dealt with by well-known specialists, Mr. 

 Oliver's share in the work being chiefly editorial. The 

 fundamental chapter on the telescope and observatory, 

 which is full of practical information, is appropriately 

 contributed by Sir Howard Grubb. Mr. Maunder con- 

 tributes an instructive chapter on the sun ; Mr. Gore deals 

 with variable stars, of which an admirable list is given ; 

 and Mr, Denning gives directions to those who are 

 anxious to distinguish themselves as comet-discoverers. 

 The chapter on the moon is very detailed, and, with the 

 index map, will be of great service to observers of our 

 satellite. Special stress is in all cases laid upon the im- 

 portance of adapting the ends to the means. The book is 

 thoroughly practical throughout, and Mr. Oliver deserves 

 the thanks of all who are interested in the progress of 

 astronomy, for bringing together such an excellent series 

 of papers. Celestial spectroscopy and photography are 

 reserved for a forthcoming volume, which we sincerely 

 hope will not be behind the one already issued. 



LETTERS TO THE EDITOR. 



\The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take 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 communi- 

 cations.'] 



The Micromillimetre. 



Prof. Rijcker's note in Nature, of February 23 (p. 388) 

 induces me to ventilate a suggestion in nomenclature which, 

 among other advantages, might reconcile the practice of botanists 

 and biologists with the C.G. S. system by leading to the disuse 

 of the prefixes mega- and micro- in favour of self-significant 

 prefixes. 



It is not improbable that, in spite of Prof. Riicker's protest, 

 the arbitrary definition of the prefixes mega- and micro-, laid 

 down in the C.G.S. system may come, or continue, to be dis- 

 regarded in different departments of science, until the ambiguity 

 thence arising necessitates their disuse, as the disuse of the words 

 billion, trillion, &c., is necessitated by their different senses in 

 English and French. Be this as it may, it is certainly desirable 

 that those who are not in the daily habit of speaking of 

 megohms, megadynes, microm'tres, Sic, should be saved the 

 necessity of recalling, or hesitation in realizing, the precise 

 meaning of the prefixes. 



Instead of denoting decimal multiples by Greek, and decimal 

 parts by Latin, prefixes to the name of the unit, let the multiples 

 be denoted by the addition of a termina ion -n (say), with a 

 suitable vowel, and the parts by that of a termination -t (say), 

 and let the order of multiples and parts alike be denoted by 

 numeral prefixes indicating ths pozuer often by which the unit is 

 multiplied or divided, or, what is the same thing, the distance 

 of the digit denoting it from the units digit. 



Thus, starting from the metre, instead of the scale — 



f decametre, hectometre, kilometre, &c , 

 me '"^ 1^ (jecimetre, centimetre, millimetre, &c., 



we might adopt the following : — 



( metron (or monometron), dimetron, trimetron, tetra- 

 metron, &c., 

 metret (or monometret), dimetret, trimetret, tetra- 

 [ metret, &c. 



Then the micrometre (the botanists' micromillimetre), would 

 become the hexametret ; the megohm, the hexohmen ; the mega- 

 dyne, the hexadynen, &c. 



As an aid to the memory, such a system would be valuable, 

 reinforcing the visua' memory, which has (I think) in many 

 cases to be relied on, by a corresponding oral reading. Thus, 

 the unit of attraction of gravitation in the C.G.S. system is 

 about 6| X 10"* dynes that is, in the proposed language, 6i 



metre 



