November 21, 1895] 



NATURE 



55 



E-TOOst part, at considerable 'depths, as in the depressions of 70 to 

 ■90 fathoms found in Loch Fyne. 



I notice that this letter has now been reported in the Fish 

 Trades Gazette, and, fearing lest the statement should go further 

 nchallenged, I should like to suggest that the experiences of 

 Jrady, Herdman, and Scott cannot be set aside lightly. I in no 

 ly doubt the statement that cojiepods are abundant on the mud 

 70 fathoms, but from the evidence given it is surely unnecessary 

 conclude that such depths are the natural feeding-ground of the 

 herring. In Loch Fyne, after continued calms, I have seen con- 

 gregations of Calantis finmarchicus floating in belts of dull 

 opaque yellow, or left by the tide in such masses as made it 



Sssible to scoop them up in one's hand. It is far from pro- 

 ble, I fancy, that Mr. Turbyne's deep-water forms were more 

 abundant, and yet the question is not merely one of the vertical 

 range of copepoda. That herrings descend to a considerable depth 

 seems true. That their eggs will hatch at as great a depth of 95 

 fathoms, the late Mr. George Brook and I proved in Loch Fyne. 

 If at certain seasons they tend to congregate in deep areas, it is 

 fortunate that, as Mr. Turbyne shows, a favourite food is to be 

 had ; yet when the copepods were as I describe, I saw the 

 herrings being caught near the surface — I spent many nights off 

 Skippness Point and in Kilbrennan Sound — and the fishermen 

 told me that beneath such floating belts of copepods, closely 

 packed shoals are usually to be met with. The fishes taken 

 were gorged with Calanus and a few other species, such as 

 Temora longicornis and Dias longireniis. The so-called disease 

 of gut-pock was then prevalent. This I attributed, rightly or 

 wrongly, to excess of oil brought into the intestinal tract by the 

 copepods. The oil extruded plentifully from the anal openings. 

 This, then, is evidence of herrings feeding freely on a plethora of 

 copeixxis near or on the surface ; evidence of a similar kind to 

 that of Mr. Turbyne's, yet with an opposite deduction. 



W. L. Calderwood. 

 Napier-road, Edinburgh, November 11. 



MacCuUagh's Theory of Double Refraction. 



It is quite clear that Mr. Larmor has misapprehended the 

 purport and object of the criticisms in my letter to Nature of 

 October 3. 



I have not attempted to discuss the question whether or not 

 "a gyrostatic ether may be constructed which will function (j/r) 

 according to MacCuUagh's optical scheme." What I have said, 

 is that the theory advocated by Mr. Larmor violates the 

 l)rinciple of angular momentum, which is a totally different thing. 

 I have also carefully avoided making use of any such phrases 

 as " ordinary elastic matter," " elastic solid matter," and the like ; 

 for the employment of such expressions with reference to a 

 quasi-fluid medium of extreme rarity, such as the ether is sup- 

 posed to l)e, is altogether misleading, and is responsible for a 

 good deal of misconception upon the subject. 



Ever since the undulatory theory was accepted by the scientific 

 world, the object of mathematicians has been to endeavour to 

 form a conception of a medium whose properties are such, that 

 optical phenomena may be deduced therefrom by dynamical 

 ])rinciples and methods ; and in order that any attempts should 

 be successful, it is necessary for the medium to possess inertia, 

 and also to be capable of resisting deformation. The latter 

 condition requires that the forces on any right solid element, 

 which are due to the action of contiguous parts of the medium, 

 should consist of nine stresses. Now, whatever physical 

 properties we may ascribe to the ether, or whatever the mathe- 

 matical form of the equations may lie which connect the nine 

 stresses with the quantities upon which deformation depends, the 

 ]jrinciple of angular momentum requires that three relations 

 should exist between the six conjugate stresses, except in the 

 two special cases I have referred to in my former letter ; that is 

 to say, unless the medium is a g)rostatic one, or unless it is 

 under the action of some system of forces (mutual or otherwise) 

 whose action upon an element consists of a couple as well as a 

 force. 



Mr. Larmor appears to think that a satisfactory solution can lie 

 obtained by postulating an energy function, and deducing the 

 •equations of motion and the boundary conditions by the principle 

 of^ least action ; but this view is fallacious. For if some particular 

 form of the energy function were assumed, and were found to 

 lead to results which violate the principle of angular momentum, 

 the theory would be dynamically unsound, and the results would 

 represent some impossible form of motion. To write down 



NO. 1360, VOL. 53] 



certain mathematical expressions, and to perform certain mathe- 

 matical operations, do not constitute a satisfactory theory, unless 

 it can be shown that all the results furnish a consistent scheme 

 in which none of the fundamental principles of dynamics are 

 violated. 



Mr. Larmor also suggests that possibly the energy function 

 of a gyrostatic medium might be modified' in such a manner that 

 MacCuUagh's equations might be deduced therefrom by means 

 of the principle of least action, and that it would be interesting 

 and instructive to establish this in detail. But why has this 

 not been done ? If a mathematical investigation of this kind 

 would be unsuitable for Nature, the same objection would 

 certainly not apply to the Philosophical Transactions. 



I shall defer entering fully into the subject of magnetic action 

 on the present occasion ; but in the paragraph marked (2), Mr, 

 Larmor has altogether failed to deal with the objection which I 

 advanced, viz. that his theory is open to exactly the same defect 

 as my own, inasmuch as it makes the tangential component of 

 the E.M.F. discontinuous at an interface. What, according to 

 Mr. Larmor's view, is the mathematical expression for the 

 E. M. F. in a magnetised medium ? What is the physical inter- 

 pretation of the boundary condition referred to in my second 

 letter ? Does he propose to modify Maxwell's equations con- 

 necting the E.M.F. with the electric displacement ? If he does, 

 he ought to have stated the fact, explained the nature of the 

 modifications he proposes, and his reasons for adopting them. 



In conclusion, I fail to see that Mr. Larmor has made any 

 adequate reply to my criticisms. To do this he must show ( I ) that 

 his resuscitation of MacCuUagh's theory does not violate the 

 principle of angular momentum ; (2) that his magnetic theory 

 makes the tangential component of the E.M.F. continuous at 

 an interface. A. B. Basset. 



Fledborough Hall, Holyport, Berks, November 12. 



The Nomenclature of Colours. 



The letter of Mr. Herbert Spencer in your issue of August 29, 

 suggesting the use of a nomenclature to be used in connection 

 with a scheme of colour standards, on a plan similar to that used 

 by sailors in boxing the compass, is interesting, both because of 

 its obvious appropriateness and because, as Mr. Spencer suggests 

 may be the case, it has occurred to others. In a paper read 

 before the American Association in 1890, I suggested the same 

 idea, but at the same time stated that this would probably give 

 more hues between the six standards than would be convenient 

 in view of the uncertainty that will almost surely result in the 

 mind of the common observer as to just which term should be 

 applied in any one particular case. In view of this, I thought 

 it wise to make the number of hues between the standards 

 smaller than the compass chart would give. Those who have 

 applied the standard colours to educational work have found 

 this desirable. 



The Milton Bradley Company, in their echicational series of 

 colour papers, have introduced only two hues between the 

 standards. This makes the series R OR RO O YO OY Y GY 

 YG G, &c. Mr. Louis Prang, of Boston, some two years since 

 published a small pamphlet, proposing to make the series for 

 common use R RRO ORO O OYO ^'O YYO Y, &c. On the 

 whole, I think it wise to adopt the smaller number for ordinary 

 use, and interpolate others for more delicate discriminations. 

 A plan which seems to me a very practical one is the follow- 

 ing, in which the hues represented by the symbols in capitals 

 are for ordinary use, and those in small type for more careful 

 discrimination : R orr ror OR oor rro RO oro roo O yoo oyo 

 YO yyo ooy OY yoy oyy Y, &c. Translated into words, this 

 would be RED, orange-red red, red orange-red, ORANGE 

 RED, orange orange-red, red red-orange, RICD ORANGE, 

 orange red-orange, ORANGE, &C. Whatever method is used, 

 however, does not in any way affect the value of the scheme of 

 definitely determined standards of colour as proposed in my 

 paper. J. H. Pili.sbury. 



Stoneham, Mass., U.S.A., Noveml)er 7. 



The Scientific American, of New York City, in its issue of 

 October 19, 1895, quotes from a letter to Nature (vol. 

 Hi. p. 413), written by Mr. Herbert Spencer, suggesting that a 



1 The theory of the mixed transformation of Lagrange's equations, which 

 leads to the energy function alluded to, w.-is first given by myself in 1887. 

 i^c Proc. Caiiib. Phil. Soc, vol. vi. p. 117, and "Hydrodynamics," vol. i. 

 P- 173-) 



