August 17, 1888.] 



SCIENCE. 



thus coinciding with the tninsniitted part of the first pencil ; and 

 the two ])encils are thus Ijrought to ' interfere.' ' A little consider- 

 ation will show that this arrangement is exactly equivalent to an 

 air-film or plate between two plane surfaces. The interference 

 phenomena are therefore the same as for such an air-plate. 



If the virtual distance between the plane surfaces is small, white 

 light may be employed, and we have then colored fringes like New- 

 ton's rings or the colors of a soap-film. If the distance exceeds a 

 few wave-lengths, monochromatic light must be employed. We 

 may confine our attention to the case of two parallel surfaces. 

 Here it can readily be shown that the fringes are concentric cireles, 

 the common axis of the rings being the normal passing through 

 the optical centre of the eye or telescope. Further, they are most 

 distinct when the eye or the telescope is focused for parallel rays. 

 In any other case we are troubled with the same perplexing 

 changes of form and position of the fringes as already noted. 



If, now, one of the mirrors have a motion normal to its surface, 

 the interference rings expand or contract ; and. by counting the 

 fringes as they appear or disappear in the centre, we have a means 

 of laying off any given distance in wave-lengths. 



Should this work of connecting the arbitrary standard of length 

 — the yard or the metre — with the unalterable length of a light-wave 

 prove as feasible as it is hoped, a next step would be to furnish a 

 standard of mass based upon the same unit. 



Suppose a cube, ten centimetres on a side, with surfaces as 

 nearly plane and parallel as possible. Next suppose a testing-in- 

 strument made of two parallel pieces of glass, whose inner surfaces 

 are slightly farther apart than an edge of the cube. The parallel- 

 ism and the distance of these surfaces can be verified to a twentieth 

 of a wave. Now apply this testing-instrument to the three pairs of 

 surfaces of the cube, and determine their form, parallelism, and 

 distance to the same degree of accuracy. We have thus the means 

 of measuring the volume of a cubic decimetre with an error less 

 than one part in a million.. 



It does not seem extravagant to say that by some such plan as 

 this we may obtain a, standard kilogram which will be related to 

 the standard of length with a degree of approximation far exceed- 

 ing that of the present standard. The apparatus can also be used 

 in the manufacture of plane surfaces, and in the measurement of 

 ■co-efficients of expansion. 



For all measurements of refraction and dispersion, — for solids 

 and liquids as well as for gases, — and in the determination of the 

 wave-length of standard lines, the accuracy of the measurement of 

 absolute wave-lengths will depend on the accuracy with which the 

 fixed distance can be compared with the standard metre ; and this 

 may be estimated as one part in two million. 



The results of the remarkable work of Rowland do not claim a 

 much greater degree of accuracy than one part in half a million for 

 relative determinations ; while the elaborate research of Bell on 

 absolute wave-lengths claims but one in two hundred thousand. 



It may possibly help to realize the very considerable superiority 

 ■of this instrument over the grating — at any rate, for the class of 

 work in question — if I recall to your attention the fact that by its 

 means it has been possible to show that the red line of hydrogen is 

 a very close double. 



Closely connected with the preceding investigations is the study 

 of the effect of the temperature, thickness, and density of the source 

 on the composition of the radiations, as shown by the symmetrical 

 or unsymmetrical broadening of the spectral lines, and the conse- 

 quent shifting of their mean position. This question has quite re- 

 cently been taken up liy H. Ebert, and the results he has already 

 obtained are very promising. Ebert has established two conclu- 

 sions, which, if verified, are of the greatest importance : namely, first, 

 that the chief factor in the broadening of the spectral lines is the 

 increase in density of the radiating body ; second, that the broaden- 

 ing, in all the cases examined, is unsymmetrical, causing a displace- 

 ment of the line toward the red end of the spectrum. The impor- 

 tance of these conclusions, in their relation to the proper motions 

 of the heavenly bodies and their physical condition, can hardly be 

 overestimated. The value of results of this kind would, however, 

 be much enhanced if it were possible to find a quantitative relation 



between the density of the radiating substance and the nature of 

 its radiations. In the case of hydrogen enclosed in a vacuum tube 

 this could readily be accomplished. It may, however, be objected 

 that it would be difficult in this case to separate the effects of in- 

 creased density from those due to the consequent increase in the tem- 

 perature of the spark. The problem of thelemperatureof the electric 

 discharge in rarefied gases is one which has not yet been solved. In 

 fact, it may seriously be questioned whether in this case temperature 

 has any thing to do with the accompanying phenomena of light ; and 

 it appears to me much more reasonable to suppose that the vibratory 

 motion of the molecules is not produced by collisions at all, but 

 rather by the sudden release of tension in the surrounding ether. 



BOOK -REVIEWS. 



nd plane parallel plate of the s.-»mc thickn 

 on) in the path of the first pencil. 



nd inclination is placed (fo 



The Philosophy of Kant. By JrjHN Watson. New York, Mac- 

 miUan. 8". S'75- 



Till'; present volume consists of a number of extracts from 

 Kant's principal works, — 'The Critique of Pure Reason,' 'The 

 Metaphysic of Morality," ' The Critique of Practical Reason,' and 

 ' The Critique of Judgment,' —and is intended for the use of teach- 

 ers of philosophy. Undoubtedly the study of Kant is the best in- 

 troduction into modern philosophy, and a powerful means of 

 guarding students from falling into a shallow materialism or 

 positivism. The extracts are well selected, and the difficult task 

 of rendering Kant into intelligible English svithout altering the 

 character of his style too much has been skilfully solved. The 

 book is an enlarged edition of the author's ' Extracts from Kant's 

 Writings,' which was originally printed for the use of his own 

 students. Professor Watson says that he found by experience the 

 results obtained by means of lectures on philosophy very unsatis- 

 factory, as the students did not learn to think for themselves : 

 therefore he adopted the plan of supplementing his lectures by the 

 study of the writings of various philosophers. This is the same 

 method which is so successfully followed at German universities in 

 what are called ' seminaries.' The teacher who will take this 

 course will find Watson's book very useful and convenient, as it 

 contains the salient points of Kant's philosophy. 



Latin Accidence and Exercises. By W. Welch and C. G. DUF- 

 FIELD. London and New York, Macmillan. 24". 40 cents. 



This book is intended as an introduction to Macmillan 's ' Ele- 

 mentary Classics.' The principles on which the authors' plan is 

 based are a thorough and accurate mastery of the elements of the 

 Latin language, and the putting into intelligent practice at once 

 what has been learned, thus avoiding as much rote-work as pos- 

 sible. The examples have been taken largely from the ' Public 

 Schools Latin Primer,' as the latter is most widely used in the 

 higher forms. The authors do not deem it desirable that beginners 

 should learn the conjunctive mood, which, for this reason, has 

 been added in small type at the end of the ' Accidence.' The 

 book is intended to be mastered in two terms. 



Elementary School Atlas. By J. B.vrtholo.MEW. London, 

 Macmillan. &". 30 cents. 



The publication under review belongs to Macmillan's Geo- 

 graphical Series, edited by A. Geikie, who promoted the interests of 

 teaching geography so well by his well-known essay on this sub- 

 ject. As might be expected, the atlas represents a great improve- 

 ment upon the ordinary English elementary school-maps, the 

 material which is embodied in the maps being carefully selected, and 

 the abominable relief-plate printing being at last discarded, a clear 

 lithograph takitig its place. The atlas contains twenty-four 

 maps or plates. The first shows a number of hemispheres: the 

 northern and southern (land and water) and the European and 

 South American. We would gladly miss the last, as it is intended 

 only to show the central position of Europe. The second map is 

 named ' Europe, illustrating Geographic Terms.' This map must 

 be considered a failure, as it attempts the explanation of geographic 

 terms, instead of by means of objects, by that of a highly and 

 wrongly generalized map. The following plate, which illustrates 

 the mapping of a landscape and the influence of reduction, ought 



