198 



NA TURE 



[July 2, 1908 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his corresponde}its. Neitlier can he undertake 

 to return, or to correspond with tile writers of, rejected 

 manuscripts intended for this or any other part 0/ Nature. 

 No notice is taken of anonymous communications.] 



Uniformity in Lantern-slides. 



In illustrating- a scientific lecture it is important that 

 lantern-slides produced from photographic negatives should 

 be of uniform density, and should well exhibit the details 

 which they are intended to illustrate. To blame the 

 lanternist for faults which are not his own may give relief 

 to the lecturer's feelings of disappointment, but this plan 

 does not conduce to the success of the lecture. 



For some time past I have been experimenting on photo- 

 metric methods of comparing exposures in printing from 

 negatives of widelv different density, and 1 find that if 

 the tests are made with sufficient care the results exhibit 

 a remarkable degree of uniformity. 



A simple photometer can be made of a sheet of white 

 cardboard folded into the shape of an isosceles prism or 

 double inclined plane, the faces of which are illuminated 

 by sources of light placed on opposite sides of the prism, 

 the distances of the sources being regulated according to 

 the law of inverse square. Now let two negatives under 

 comparison be placed side by side in front of the two 

 faces, and examined by the transmitted light reflected 

 from the cardboard. Then, when the negatives look to be 

 of the same density, their exposures will be proportional to 

 the illuminations of the faces, and can be easily compared. 

 If the negatives differ in contrast, this difference will be 

 at once evident on adjusting the illuminations, and either 

 the necessary modifications of treatment can be decided on 

 or badly contrasted negatives can be set aside for intensifi- 

 cation or omitted from the series. By a method identical 

 in principle with the above I have been successful in test- 

 ing the development of negatives and in calculating 

 exposures in bromide enlarging. Two negatives exposed 

 in succession with calculated exposures of twenty-five 

 seconds and fifty minutes have given under similar develop- 

 ment equally good prints of almost exactly the same dark- 

 ness. G. H. Bkvan. 



A Probable New Fluting in the Spectrum of 

 Magnesium Oxide. 



There appears to be a well-marked, though faint, fluting 

 in the spectrum of magnesium oxide which has not been 

 hitherto recorded, consisting of seven principal edges and 

 several fainter lines. No mention of it has been found 

 anywhere, and Prof. Kayser, who has seen a photograph, 

 says that it is unknown. If so, this is probably due to the 

 fact that it would be quite invisible against even a weak 

 continuous spectrum. 



The wave-lengths of the principal edges have been deter- 

 mined by comparison with lines of zinc, cadmium, and 

 manganese, and are approximately as follows : — 4823, 

 4819, 4810, 4801, 4791, 4780, 4771. The first of these is 

 very faint, and although almost coincident with the Mn 

 line at 4823, appears to have a slightly greater wave- 

 length, and is probably not due to Mn as impurity. The 

 edge at 4780 is rather diffuse, and two faint lines have 

 been measured between it and 4791. Between 4771 and 

 4731, five lines have been measured, which may also belong 

 to the system. 



The fluting is obviously related to that beginning at 

 5007 : the spacing between the edges is of the same order, 

 and it is only well seen when the latter is very intense. 

 .Although first observed about twelve months ago, it was 

 only successfully photographed last February. Some of 

 the negatives also show that the series of faint, fine lines 

 on the less refrangible side of the violet magnesium triplet 

 extends much further into the visible spectrum than cata- 

 logued by Eder and Valenta. E. E. Brooks. 



Leicester Municipal Technical School, June 18. ^ 



The Halos round Zircons in Biotite. 



With reference to the action of radium on glass, and its 

 remov^ by exposure to sunlight, the following unintentional 

 experiment may possibly be of interest. Manv years ago 



NO. 201S, VOL. 78] 



I had a section of a piece of granite prepared, and then 

 .inother after the stone had been made red hot in an 

 ordinary bright fire. 



In the unhealed rock the zircons in the brown micas 

 show good halos, and these have not been obliterated by 

 the strong heating. This may be worth mentioning, as 

 the experiment may possibly not have been attempted by 

 anyone else, either from lack of motive or the difficulty of 

 getting a good slice after the rock has been made brittle 

 by the heat. A. R. Hu.nt. 



Southwood, Torquay, June 20. 



LORD KELVLN'S PHILOSOPHY.^ 



Explanation in Terms of Force or of Motion? Action 

 across Empty Space or through a Medium ? 



ONE of the most interesting and important outcomes 

 of last year's meeting of the British Association ' 

 at Leicester was the declaration by Lord Kelvin, ciuring 

 a memorable discussion on the constitution of the 

 atom, in Section A, that he had found it necessary 

 to abandon the attempt to contemplate the material 

 universe explicitly in terms of aether and motion, and 

 for his own part preferred to resort to the Boscovich 

 doctrine of centres of force acting on each other 

 according to some curiously comple.x law, without 

 specific attention to the hypothetical medium in which 

 such forces may e.\ist. 



Now undoubtedly these ancient postulates of matter 

 and force represent the dynamical method first made 

 feasible by Newton's achievement in celestial physics, 

 whereby phenomena were correlated by unexplained 

 particles of matter acted upon by unexplained forces, 

 of statical origin and unknown mechanism, according 

 to a specified law of distance. This was how Newton 

 successfully solved the problems of gravitation, and 

 constructed the working theory of astronomy ; but 

 it had been hoped, and by some is still hoped, that 

 the time had now come for seeking to represent, in 

 terms of something simpler and more fundamental, 

 the nature of matter and the origin or inner 

 mechanism of its various forces. 



The most powerful and hopeful lever wherewith 

 to attack this great philosophical problem was the 

 kinetic theory of elasticity and rigidity, introduced 

 by Lord Kelvin himself. By this means it has been 

 hoped to express force in terms of the still simpler 

 conception of motion ; in fact, to explain all the forces 

 with which physicists have to do — electrical and 

 chemical attraction, elasticity, magnetism, cohesion, 

 and perhaps gravitation — in terms of the internal 

 motions of a universally connecting fluid plenum. 



But now the question arises, is it at all certain that 

 the material universe can really be understood in 

 terms of motion alone — motion of an all-pervading' 

 continuous fluid known as the a?ther of space? .-\nd 

 would such a solution be satisfactory ? 



To many it has seemed that this reduction to sim- 

 plicity was the closest approach to ultimate explana- 

 tion and unification that could be hoped for in the 

 domain of mathematics and physics; and during the 

 last half-century many steps, apparently in the direc- 

 tion of such an achievement, have been taken by the 

 leaders in these branches of human knowledge. 



The mathematical foundation was laid by Helm- 

 holtz, when he reduced rotational or vortex motion 

 in perfect fluid under the domain of mathematics; 

 it was followed up bv Lord Kelvin's kinetic or gyro- 

 static theory of elasticity and rigidity; so that mathe- 

 maticians, such as FitzGerald, Heaviside, Larmor, 



1 Being tlioughts siiesested by the meeting of llie Mathem.itic.il and 

 Physical Section of the British Association at Leicester in August. 1007 ; and 

 referred to in Sir Oliver Lodge's recent Presidential Address to the Karaday 

 Society, May 26, 190S. 



