October 22, 1891] 



NA 7 URE 



597 



* In Holland the tests are made by experts. In England 

 they are applied by persons who, however well they 

 may be qualified to examine candidates in navigation 

 and seamanship, have certainly no locus standi in the 

 matter of reporting upon the perfectness, or otherwise, of 

 a man's visual organs. 



The tests themselves that these navigation examiners 

 have to apply are far from being perfect. They are 

 established upon a wrong principle. Candidates are 

 made to name colours, and according to the Parliament- 

 ary Report of 1887 "the only reasons for which they 

 are reported as having failed are inability to distinguish 

 red from green, and either from black by daylight, and 

 red from green and either from ground glass by artificial 

 light." 



Candidates are first required to give correct colour 

 names to a series of eight cards coloured black, red, 

 green, pink, drab, blue, white, and yellow, respectively. 

 A candidate is passed, however, if he names correctly 

 the first three. 



The second test consists in naming the colours of 

 glasses some eleven in number, viz. ground glass, stan- 

 dard red, pink, three shades of green, yellow, neutral tint, 

 two shades of blue, and white. The candidate need, how- 

 ever, only name the ground glass, the standard red, and 

 the standard green. 



Clearly, with such tests as these, the colour-blind may 

 easily escape detection. 



The Board of Trade return relative to colour tests for 

 the year ending May 31, 1891, shows that out of 4688 can- 

 didates who presented themselves for masters' and mates' 

 certificates, 31 were rejected on account of deficient 

 colour sense. That these should be rejected after serving 

 an apprenticeship to the sea, is manifestly unfair. The 

 test should be applied at the commencement of their 

 nautical career, and not when the initial stage is 

 passed. Four of the 31 were reported as passing on sub- 

 sequently undergoing examination, although medical 

 expert opinion is emphatic in stating that colour-blind- 

 ness is absolutely incurable. Perhaps it may be that the 

 examiners were disposed, by their leniency in passing 

 young men whose previous " failure in colours " proved 

 them colour-blind, to atone in some slight form for the 

 bad system which allows lads to spend the best years of 

 their life in mastering the irksome details of a profession, 

 before it informs them that they are visually unfitted for 

 it. It is to be hoped that the investigation into the 

 whole system of colour-testing at present being con- 

 ducted by a committee appointed by the Royal Society, 

 may lead to thorough and efifective reforms. 



T. H. BiCKERTON. 



ON VAN DER WAALS'S TREATMENT OF 

 LAPLACE'S PRESSURE IN THE VIRIAL 

 EQUATION : A LETTER TO PROF. TAIT. 



MY DEAR PROF. TAIT,— 1 gather from your letter 

 of September 28 (Nature, October 8, p. 546) that 

 you admit the correctness of Van der Waals's deduction 

 from the virial equation (i) when the particles are infinitely 

 small, in which case 



(^+^0 



i2wV2 



(0 



a representing a cohesive force, whose range is great in 

 comparison with molecular distances ; and (2) when, in 

 the absence of a cohesive force, the volume of the particles 

 is small in comparison with the total volume v, in which 

 case the virial of the repulsive forces at impact gives 



p(v - h) == I'S.mN^ (2) 



For hard spherical masses, the value o{ b is four times 

 the total volume of the sphere. But you ask, " How can 



NO. II 47, VOL. 44] 



the factor (^ — (J)/?', which Van der Waals introduces on 

 the left (in the first case) in consequence of the finite 

 dia eters of the particles, be justifiably applied to the 

 term in K (or alv-) as well as to that in/ .' " 



Ifi my first letter I desired to avoid the complication 

 entailed by the consideration of the finite size of the 

 particles ; but it appears to me that the argument ihere 

 given (after Van der Waals) suffices to answer your 

 question. For, if the cohesive force be of the character 

 supposed, it exercises no influence upon any particle in 

 the interior, and is completely accounted for by the addi- 

 tion to/ of alv". In so far, therefore, as (2) is correct 

 when there is no cohesive force, the effect of such is 

 properly represented by 



{p^^^{v-b) = \^m'^'^ 



. . (3) 



in which b is to be multiplied by (t/v% as well as by/. 

 Yours very truly, 

 October 13. Rayleigh. 



NOTES. 

 At the Royal College of Physicians, on Monday, when the 

 Harveian Oration was delivered by Dr. W. H. Dickinson, the 

 Baly Medal was given to Prof. Michael Foster for distinction in 

 physiology ; the Morgan Medal to Sir Alfred Garrod for dis- 

 tinction in clinical medicine. 



Dr. Dickinson, in the Harveian Oration, presented an 

 admirably clear and vigorous account of Harvey's great 

 discovery, and of the scientific results to which it has led. 

 The earliest and most important of these results was the 

 completion of Harvey's work by the discovery of the capil- 

 lary system by Malpighi, who was born in the year in which 

 Harvey published his famous treatise. " Harvey," said Dr. 

 Dickinson, "had never seen a capillary, nor did the state of the 

 microscope in his time allow of it. He was fain to conclude 

 that the blood passed from the arteries to the veins partly by 

 anastomoses but mainly by percolation, as water, to quote his 

 own illustration, percolates the earth and produces springs and 

 rivulets. Had it been possible, we may imagine the delight 

 with which he would have witnessed the completion by vessels 

 of his circular route." Dr. Dickinson also referred, among 

 other results of Harvey's discovery, to embolism, and to our 

 knowledge of inflammation, or at least as much of it as con- 

 cerns the capillaries. In conclusion, he said : — " Knowledge 

 has been advancing since Harvey's time in many and inde- 

 pendent lines ; the achievements of Bell, Bright, and Addison 

 had no direct connection with his, but it is not too much to 

 assert that the medicine of to- day is scarcely less permeated 

 with the results of Harvey's discovery than is the human body 

 with the circulation he discovered. It does not make him 

 small to say that what he found out must have come to light 

 had he never lived. If Columbus had not discovered America 

 some one else must have done so before now. The law of 

 gravity might even have been revealed in the fulness of time to 

 another if not to Newton. Bat the discoverer is before his 

 time ; in this lies one measure of his praise ; another, and a 

 more important one, is in the results of his discovery." 



The Electrical Exhibition, to be opened at the Crystal 

 Palace on January i next, promises to be one of great interest 

 and importance. The requests for space — which already exceed 

 a total of 200— include electric lighting plants for country and 

 town houses, for mines, for steamships, for railway trains, and 

 even for private carriages. There are also included the newest 

 forms of motors, generators, accumulators, and other machinery 

 employed for producing and storing electricity. Several of the 

 more important exhibits at the Frankfort Exhibition will be 



