572 



NATURE 



[Oa. lo, 1889 



he is within 10 yards, and walking steadily on. There 

 lies a stone, on which you had laid your caross and other 

 things, when making ready to enter your shooting-screen ; 

 the beast has come to it, he sniffs the taint of them, tosses 

 his head up wind, and turns his huge bulk full broadside 

 on to you. Not a second is to be lost. Bang ! and the 

 bullet lies well home under his shoulder. Then follows 

 a plunge and a rush, and the animal charges madly about, 

 making wide sweeps to right and left with his huge horn, 

 as you crouch down still and almost breathless, and with 

 every nerve on the stretch. 



"He is off; you hear his deep blowing in the calm 

 night ; now his gallop ceases. The occasional rattling of 

 a stone alone indicates that he is yet a-foot ; for a moment 

 all is still, and then a scarcely audible 'sough' informs 

 you that the great beast has sunk to the ground, and that 

 his pains of death are over." 



The author has long since been so well known as a 

 gold medallist and leading member of the Royal Geo- 

 graphical Society, and for his important contributions to 

 anthropology and other departments of science, that it 

 would be superfluous at this time to dwell on the value of 

 his explorations, compared with those of the ordinary and 

 less-gifted traveller. His exceptional aptitude for what 

 may be called professional travel is well exemplified at 

 pp. 180-82 of the volume now republished, in which, 

 among other useful hints, he gives plain and practical 

 instructions for selecting the best sort of travelling com- 

 pass and checking distances and directions. His manual 

 on the " Art of Travel " has for many years been a 

 standard work of reference ; while no one who reads, in 

 the " Narrative of an Explorer," his amusing record of 

 " a series of observations " taken by sextant upon the 

 figure of a Hottentot lady — with results worked out " by 

 trigonometry and logarithms " — can affirm that his sense 

 of humour has been blunted by scientific pursuits. 



A republication of "Vacation Tours in i860 and 1861 " 

 — papers by Sir George Grove and the late Mr. W. G. 

 Clark, added to one by Mr. Galton — enhances the value of 

 this new accession to the Minerva Library. 



OUR BOOK SHELF. 



Practical Photometry: a Guide to the Study of the Measure- 

 ment of Light. Bv W. J. Dibdin. (London : Walter 

 King, 1889.) 



This work forms a good practical text-book on the art of 

 photometry, which, both scientifically and commercially, 

 is becoming more and more important. It contains a 

 comprehensive account of the various methods in daily 

 use, so that the student, when he finds that he is dealing 

 with instruments and methods unfamiliar to him, may 

 turn to this book as a guide to the many precautions 

 necessary to insure accurate results. The first few chap- 

 ters deal with the history and principles of photometry, 

 together with horizontal, radial, and jet photometers, 

 and diagrams are given of the determinations of the 

 quantity of light afforded in all directions horizontally 

 by three classes of flames tested at every 10"", and also 

 of Dr. Pole's method of expressing the illuminating 

 power and rates of consumption per hour of fifteen- 

 candle gas. In chapters vi. and vii. we have a discus- 

 sion on the various standards of light which have been 

 and aie still in use, followed by the numerous proposed 

 substitutes, such as Harcourt's pentane, Sugg's sixteen- 

 candle argand, Methuen's screen, &c. The apparatus 

 necessary to check and measure the flow of gas to the 

 standard burner is given in chapter viii., with detailed 



descriptions. Chapters ix. and x. treat of" The Examina- 

 tion and Adjustment of a Gas-testing Photometer" and 

 " Colour Photometry," the latter dealing with methods of 

 estimating the colour and intensity of the illumination of 

 fabrics, &c. Lastly, in chapter xi., on " Stellar Photo- 

 metry," the author gives an account of the methods 

 employed by Sir John Herschel, ZoUner, and others, con- 

 cluding with a description of a method proposed by 

 himself. 



The appendix contains some useful pieces of miscel- 

 laneous information, and tables of illuminating power of 

 sperm candles, candle corrections, &c. The work is well 

 illustrated with numerous woodcuts of the various instru- 

 ments employed. 



LETTERS TO THE EDLTOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 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 communications. ] 



The Testing of Colour Blindness. 



The important article in Nature of September 5 (p. 438) 

 will have been read by all friends of education with deep and 

 melancholy interest. 



I desire to point out tbat the real remedy ought not to be 

 dealt with by an Act of Parliament, partly because this is a 

 makeshift, and partly because the sudden dismissal of a trained 

 seaman for a constitutional defect is so cruel that human nature 

 would get very much in the way of such an Act of Parliament. 



What is wanted is that the colour sight and other measurable 

 faculties of youths should be tested before they go out into life. 

 It is a detail of practical anthropometry. Just as no parent 

 would think of encouraging his son to become a barrister when 

 he knew that he stammered or was deaf, so no parent would 

 waste money in training his son to be an engine-driver or a 

 sailor when he knew — as he ought to know — that the lad was 

 colourblind. 



I protest against a defect of our educational system being 

 treated as a defect of legislation or of administration, and I trust 

 that the friends who seek that scientific education should have its 

 due place will move in this direction, and thus prevent the 

 cruelty to seamen and their families as well as the deaths of their 

 fellow- creatures which are at present possible from the causes 

 indicated in your article. 



I desire to ask if some of your readers will kindly furnish 

 information, through your columns, as to where the colour-tests 

 can be obtained, their cost, and the literature respecting them. 



9 King Street, Oxford, September 19. J. F. Heyes. 



Mites. 



In the grounds of the Leicester Museum there are some 

 half-dozen young lime trees, about fifteen years old, of which , 

 the trunks and under sides of the main branches are covered - 

 with an extremely thin glistening film. It seems to consist of a 

 similar material to that of which spider's web is made, but 

 spread out into a film instead of being spun into thread. When ^ 

 rubbed up it has all the appearance of spider's web similarly | 

 treated. But there are no spiders visible, except here and there an 

 Epeira, who has fixed his geometric web to some protuberance 

 on the trunk. 



The bark, however, is thickly dotted over whh small yellowish 

 mites, very similar to cheese mites in size, form, and colour. 

 Some of these mhes are creeping on the outside of the filni 

 also, and a few of them are captured in the webs of the 

 Epeira. No other trees appear to be affected except the limes, 

 and only one particular row of these. I have found a few of! 

 the mites, but no film, on other limes in the grounds, but none| 

 on horse-chestnuts or laburnums which stand nearer to the 

 affected trees. 



How has this film been produced ? and are the mites connected 

 with it as cause or effect ? F. T. MoTT. 



Leicester, September 23. 



II 



