February 1, 1892.] 



K N O W^'L EDGE 



35 



suggested to him the importauce of establishing a periodical 

 devoted to spectroscopy and astro-physics. He was loth 

 to multiply the number of astronomical journals, and it 

 was ultimately decided that the new periodical should be 

 published in conjunction with the " Sidereal Messenger," 

 which now changes its name to '• Astronomy and Astro- 

 Physics," and will appear ten times a year under the jomt 

 editorship of Profs. PajTie and Hale. The subscription 

 will be four dollars per annum. 



Notices of Boofes. 



Memun/ : Its Lo<iical Iiel(itio)is (tml L'ultirdtion. By 

 F. W. ■Edridge-Greex, M.D., F.G.S. ; 2nd Edition. 

 (Bailliore, Tindall and Cox.) According to the author, 

 memory is of two classes. There is sensory memory, 

 dealing with the impressions received by the mind either 

 from the external world or from its own processes, and 

 motor memory, dealing with the motor impulses resulting 

 therefrom — a classification based on the same principle as 

 that of nerves into sensory and motor. Memory as a 

 whole he treats as a distinct and definite faculty of the 

 mind, having for its seat a limited portion of brain tissue ; 

 the sensory division he localizes in the optic tlmlami, 

 and the motor in the curjiorn striatu, the physiological 

 basis of the memory being in each case a modification of 

 the protoplasm of the cells of the nervous centre. Using 

 the term " memory " in the broadest and most general sense 

 of retentiveness, the faculty which is the foundation of 

 . all stores of knowledge and experience, he very fully and 

 ably illustrates its differences in kind by reference to 

 many of the most ordinary details of everyday life, show- 

 ing that he has been a close and thoughtful observer of 

 human nature. The idea which runs through tlie book is, 

 perhaps, best expressed in brief by the followmg sentence : 

 " The centre for sensory memory is so arranged that every 

 impression received through a lifetime is registered in a 

 definite position and order of sequence, from the first 

 moment of a child's life to the day of his death, and all 

 sensations, perceptions, and ideas received at the same 

 time either form component parts of one impression or 

 closely associated impressions." The memory, of course, 

 consists in the revival of these impressions. The theo- 

 retical part of the subject is fully elaborated in a series of 

 clearly written and closely reasoned chapters, which are 

 extremely interesting reading whether one accepts the 

 conclusions or not. But the book has more than a 

 merely academic interest, great though that may be ; 

 Dr, Edridge-Green is intensely practical as well, realising 

 as he does tbe extreme importance to its possessor, in 

 the complex life of to-day, of a memory which can always 

 be relied upon ; and he applies his priucii^les to the con- 

 struction of a series of rules by the observation of which the 

 memory may be cultivated and improved. These show a 

 clear appreciation of the causes of the defects which are 

 most commonly experienced, and though couched in some- 

 what technical language, they are suificiently explained 

 and illustrated by actual examples which are of familiar 

 occurrence, and there will therefore be little ditficulty in 

 understanding their drift, and in putting them into 

 practice. Like most other writers on memory, the author 

 has a mnemonic system of his own, of special use as an 

 aid in the memory of numbers ; this, if somewhat cum- 

 brous and troublesome, ■will no doubt yield good results to 

 those who try it with enthusiasm, though we are of opinion 

 that each person will generally profit most by a system 

 either of his own devising, or at least of his own adapta- 

 tion. 



Handbook of the London Gcoloiikal Field Class. (George 

 Philip & Son.) This is a record of five years' work by tlie 

 above class, which, under the presidency and personal 

 conduct of Professor H. G. Seeley, has utilized Saturday 

 afternoons during the summer mouths in studying geology 

 at first hand. Excursions are made to localities which 

 can be reached by a railway ride of not more than an hour 

 from London ; the professor explains on the spot the 

 geographical and geological features of the locality visited, 

 and the students, in their turn, under the professor's 

 guidance, observe and chronicle matters of geological 

 interest, collect fossils, &c., and are thus put in training 

 which may, if they choose to follow it up, eventuate in 

 their becoming practical geologists. By this excellent 

 plan employment is found for the Saturday holiday, which 

 is at once recreative, health-giving, and instructive ; 

 habits of scientific accuracy are encouraged by the constant 

 appeal to Nature, which cannot but be of the highest 

 educational value. The handbook is compiled by members 

 of the class, under Professor Seeley's supervision, and in 

 it we have abstracts of the professor's lectures and reports, 

 between 80 and 90 in number, written by the students in 

 description of the localities visited, and illustrated with 

 sketches and sections from their own note-books. The 

 whole forms a neat little pocket volume, which will be not 

 merely of interest to the members of the class, but may 

 be usefully employed as a guide-book by any amateur 

 geologists who wish to study the cretaceous and tertiary 

 deposits of the South East of England. 



THE RELATIVE BRIGHTNESS OF THE 

 PLANETS. 



By .T. E. Gore, F.E.A.S. 



THAT the planets shine with very difl'erent degrees 

 of brightness is a fact familiar, perhaps, to most 

 people. The great brilliancy of Venus, when 

 favourably situated as a morning or evening star, 

 is well known, and has frequently given rise to 

 the erroneous idea that a new celestial visitor had appeared 

 in the sky. .Jupiter, when in opposition to the Sun and 

 high in the heavens, as it is in some years, also forms a 

 brilliant object in our midnight sky, and it is closely 

 rivalled in lustre by the " red planet " Mars, when nearest 

 to the Earth, as it will be in the autumn of the present 

 year. The difliculty of detecting the planet Mercury with 

 the naked eye, owing to its proximity to the Sun, is well 

 known. When seen, however, under favourable condi- 

 tions, this planet shines with considerable brilliancy, but, 

 as it can only be seen at its brightest for a few days in the 

 morning or evening sky a little before sunrise or a little 

 after sunset, and then only for a comparatively few minutes 

 in the twilight, it generally escapes the observation of the 

 casual observer. The " ringed planet " Saturn usually 

 appears brighter than an average star of the first magni- 

 tude, and may be easily distinguished by its dull yellow 

 colour. The light of this planet is of course considerably 

 increased when the ring system is widely open, the bright 

 rings being very luminous ; but, when the rings are nearly 

 invisible, as they are at present, the brightness of Saturn is 

 much reduced. Uranus is just visible to the naked eye on 

 a clear night when its exact position with reference to 

 neighbouring stars is known, but Neptune is quite beyond 

 the range of unaided vision. 



These differences in the relative brightness of the planets 

 are due to four causes : (1) The distance of a planet from 

 the Sun ; (2) the distance of the planet from the Earth ; 

 (3j the size of the planet ; and (4) the reflecting power 



