NATURE 



[March io, 1910 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed 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.] 



Dr. H. J. Hansen and the Copenhagen Museum of 

 Zoology. 

 On behalf of the zoologists who have signed the accom- 

 panying letter to Dr. H. J. Hansen, of Copenhagen, I 

 have been asked to send a copy to you with the request 

 that you will be good enough to publish it in Nature. 



W. T. Calman. 

 X Mount Park Crescent, Ealing, VV., March 7. 



March, 1910. 

 To Dr. H. J. Hansen, 



The University Museum of Zoology, 

 Copenhagen. 

 Dear Dr. Hansen, 



We, being some of those among the zoologists of Great 

 Britain who know and value your zoological work, have 

 heard with regret that there is a chance of your leaving 

 the Museum of Zoology in Copenhagen. We hope that 

 this is not the case ; and we more especially hope and trust 

 that you will let no circumstances turn you aside from 

 your important zoological investigations. 



The Museum of Steenstrup, of Liitken, and of Schiodte 

 is honoured by us all ; we know and honour many of the 

 fellow-workers and successors of those great naturalists ; 

 and we consider that among so many distinguished names 

 your own is by no means the least distinguished. 



To the researches that you have carried on for many 

 years, partly by yourself, partly together with your learned 

 compatriot, Sdrensen, we owe the best part of our know- 

 ledge of several important orders and families of Arthro- 

 pods ; you figure in our text-books as the leading authority 

 on such difficult groups as the Palpigradi, the Pauropoda, 

 the Cryptostemmatidas, the Hemimeridae, and the Chonio- 

 stomatidae ; and this partial list of your works is in itself 

 a proof that you have always laboured just where there 

 were real- gaps and imperfections in the common stock of 

 zoological knowledge. 



Your Monograph on the Choniostomatidae we would refer 

 to in particular as a masterpiece of delicate dissection and 

 exquisite illustration ; while in one and all of your publi- 

 cations we recognise the keenest morphological insight, and 

 an uncommon grasp of the essential principles of classifi- 

 cation. 



With our best wishes for your prosperity, we beg you 

 to receive from us this tribute to your powers and this 

 testimony of our personal regard. 



Very faithfully yours, 

 (S»^ned> A. Alcock, E. J. Allen, Ernest E. Austen, F. A. 

 Bather, G. A. Boulenger, Gilbert C. Bourne, W. T. 

 Caiman, G. H. Carpenter, Wm. Eagle Clarke, C. Clifford 

 Dobell, J. Cossar Ewart, F. W. Gamble, J. Stanley 

 Gardiner, W. A. Herdman, Svdnev J. Hickson, E. W. L. 

 Holt, E. Ray Lankester, E. W. MacBride, W. C. 

 Mcintosh, P. Chalmers Mitchell, A. M. Norman, R. I. 

 Pocock, Edward B. Poulton, R. F. Scharff, Adam Sedg- 

 wick, A. E. Shipley, Thomas R. R. Stebbing, J. Arthur 

 Thomson, D'Arcy W. Thompson, Chas. O. Waterhouse. 



Colour Blindness. 

 When reading the late case of Mr. John Trattles and 

 his colour-blindness, and when considering the discussion 

 on the value of the tests for colour-blindness in its practical 

 bearing for seamen and engine-drivers, it occurred to me 

 that there was a very simple means of enabling red-blind 

 and green-blind persons to distinguish red lights from 

 green lights, and both of these from white lights, without 

 their having to recognise the colours at all. I tested a 

 colour-blind person here first with red glass and next with 

 green glass placed in front of a cycle lamp, and he could 

 •not distinguish between the red and the green; but with 

 the aid of my device he could distinguish the red light 

 from the green light without fail, though he could not 

 see them as distinct colours. The means of effecting this 



NO. 2106, VOL. 83] 



is quite simple. I gave him suitable pieces of red and 

 of green glass. I told him to look at the white light first 

 through the red glass and then through the green glass ; 

 result, he could see the white light through either glass, 

 though he could not distinguish the colours, but when he 

 could see the light clearly through each separately of his 

 pieces of glass he knew the light was not green or red, 

 but white. 



1 then made the lamp shine through a piece of red glass, 

 and told the man I was testing to look at it first through 

 his bit of red glass and then through his bit of green 

 glass ; result, he could see the light of the lamp through 

 his bit of red glass, but could see no light through his 

 bit of green glass, and so he knew the light of the lamp 

 must be red, though he did not know its colour. Next I 

 made the light shine through a piece of green glass, and 

 when my man looked at it through his green glass he 

 could see the light clearly, but when he looked at it 

 through his piece of red glass he could not see it at all, 

 or only very, very dimly, if the green glass of the lamp 

 was a pale green and let some white through with the 

 green, but in either case he could say with certainty the 

 light was green and not red or white, and this without 

 recognising the colours as colours. 



The practical application of the above facts is simple, 

 and can be effected in a variety of manners and , in- 

 expensivel}-. For example, a sort of double eye-glass could 

 be made holding "a suitable piece of red and of green glass 

 and with a small handle, and made of a size easily to fit 

 in the pocket, or, for use at sea, it might take the form 

 of a simple night-glass with a small slider carrying the 

 coloured glasses at the eye-piece end. Anj'one can try 

 experiments in this matter with the aid of a bicycle lamp 

 and its green and red light on either side, and suitable 

 pieces of red and green glass to look through. 



Summary. — When the lamplight can be seen clearly 

 through both the red glass and the green glass separately : 

 conclusion, the light is white. 



When the lamplight is seen through the red glass and 

 not through the green glass : conclusion, the light is red. 



When the lamplight is seen clearly through the green 

 and not through the red, or only very, very dimly : con- 

 clusion, the light is green. 



It is not a case of distinguishing by colour recognised, 

 but by whether the light can or cannot be seen in each 

 case. 



I offer this suggestion in case it may be of any service, 

 and unpatented, for the free use of all who like to use it. 



Stonyhurst. H. M. 



The Meaning of lonisation. 



The columns of Nature are doubtless not the proper 

 place in which to conduct correspondence classes in 

 elementary science, but when Prof. Armstrong asks a 

 simple question surely mere courtesy demands that he 

 should receive a straightforward answer, such as Prof. 

 Walker and " A. S." have not given him. 



I imagine that nobody will quarrel with the following 

 definitions : — 



Ions are particles supposed to be present in some media 

 such that, when the medium is placed in an electric field, 

 the particles have a finite average velocity relative to the 

 medium along the direction of the field. 



" lonisation " is used in two senses : — (i) it is used to 

 denote the number of ions present in unit volume of the 

 medium ; (2) it is used to denote the process by which the 

 ions are produced. Since several such processes are 

 known, the use of the word lonisation does not connote 

 any special hypothesis as to the mechanism by which the 

 ions are produced. N. R. C. 



A Rare Crustacean. 



Yesterday my assistant, Mr. G. Pyman, found several 

 Cheirocephalus diaphanus swimming in a flooded ditch on 

 Eton Wick Common. The sunlight shining on the beautiful 

 green bodies of the males made a very striking effect, ^^"e 

 were able to catch about twenty specimens of both sexes. I 

 had never seen this phyllopod alive before, and, so far as 

 I know, it has never been recorded previously from this 

 district. The females, of brownish-purple colour, all have 



