February lo, 1898] 



NATURE 



353 



Astronomical Annual for 1898. — We have just received 

 a copy of the sixty-fifth Annual of the Belgian Royal Observ- 

 atory for 1898. The volume is similar in character to the many 

 astronomical annuals published on the continent, giving in 

 calendar form the most important astronomical events of the 

 present year, and in addition geographical data referring chiefly 

 to Belgium. Besides these there is a detailed description of 

 the Royal Observatory at Uccle, together with the instru- 

 ments and the observations made at the observatory in 1897. 



Spectrum Researches ok t; Aquil.^. — Prof A. Belo- 

 polsky has recently completed a new series of photographs in 

 connection with the spectrum of tj Aquih^ ; and his paper on 

 " Researches on the Spectrum of the Variable Star r; Aquilte " 

 appears in the Astrophysical Journal, December 1897. In ^H. 

 twelve photographs were taken corresponding to the different 

 phases of brightness, and by an *' iron comparison " on each 

 photograph, it has been possible to determine the velocity of 

 the system with respect to the sun. 



The spectrum of r; Aquilte is remarkably like that of the 

 variable star 5 Cephei, belonging to a group of which 7 Cygni 

 is the type ; therefore some of the principal iron lines contained 

 in its spectrum have been utilised in making the measures. 



From these measures, the author finds the motion of the system 

 — ~ J '^5 geographical miles, and from the curve of velocities 

 in the line of sight he concludes "that the times of minimum 

 brightness and the times for which the velocity in the line of 

 sight is zero do not coincide. For this reason 

 the changes in the brightness of the star cannot 

 be explained as the result of eclipses, and some 

 other explanation must be sought." It is very 

 remarkable that Prof. Belopolsky found this was 

 also the case with the variable star 8 Cephei. 



and, if the latter, with the frequency of the alternations. Strictly 

 speaking, Maxwell's relation only applies to the refractive index 

 for waves of infinite length, and determinations of the optical 

 refractive index, i.e. the index for electromagnetic waves of 

 about i/50,oooth of an inch, do not bear upon the question. 

 It is only the long invisible electromagnetic waves which can be 

 properly used to test the relation. 



Testing Maxwell's Relation. 



The specific inductive capacity of glass has been assigned 

 various values ranging from 27 to 9'8. The optical refractive 

 index /t is about i 5. Prof. Bose determined ^ for electric 

 vibrations of a frequency of about 10"* vibrations per second. 

 The apparatus used is shown in the diagram. 



It closely resembles an optical apjiaratus. The radiator, con- 

 sisting of two platinum beads with a platinum sphere between 

 them, and fed by an induction coil, is enclosed in the square box. 

 The rays pass through the diaphragm p to the semi-cylinder c of 

 the glass to be investigated. This semi-cylinder is turned until 

 the rays are totally reflected by the back surface. They are de- 

 tected by the receiver R, containing metallic filings, whose 

 resistance is reduced by the impact of the waves. The shielding 

 of the receiver from strong radiations is a matter of some diffi- 

 culty. Prof. Bose says : — 



" Another troublesome source of uncertainty is due to t^e 

 action of the tube which encloses the receiver. When a slanting 

 ray strikes the inner edge of the tube, it is reflected and thrown 



A SIXTH edition of Mr. Thynne Lynn's handy 

 little book on "Remarkable Comets" has just 

 been published by Mr. Edward Stanford. The 

 information in the book is completely up to date, 

 even the observed return of Winnecke's comet, 

 first seen on the present visit on January 2, 

 being recorded. Encke's comet (period 3g years) 

 may l)e expected shortly, and in the summer. 

 Wolfs comet (period 6f years) should pay us a 

 visit. 



Fig. 



-The electric refractometer : p, the plate with a diaphragm ; c, the semi-cylinder of 

 glass ; s, the shield (only one shown in the diagram); r, the receiver. 



THE REFRACTION OF ELECTRIC WAVES} 



'X'WO years ago. Prof Bose, in a communication to the .^sialic 

 Society of Bengal, described some new devices for dealing 

 with electric waves, which did much to bridge over the gulf 

 between electric waves and light waves. One of these was the 

 employment of nemalite, a fibrous variety of brucite, which has 

 the valuable property of absorbing electric waves vibrating in a 

 certain plane, and transmitting all waves at right angles to that 

 plane. It thus could be made to do for electric radiation what 

 a plate of tourmaline does for light, except that the directions 

 of absorption and transmission are reversed. Nemalite is there- 

 fore a very convenient polariser and analyser of electric waves. 

 Tourmaline also acts in the same manner (with planes reversed), 

 but not to any extent comparable with the efficiency of nemalite. 

 The apparatus was subsequently exhibited and worked before 

 the Liverpool meeting of the British Association. 



In the present papers, Prof Bose describes some experiments 

 on the refractive index of glass for electrical waves, carried out 

 for the purpose of testing Maxwell's relation K = ju'^, which 

 maintains that the specific inductive capacity for any substance 

 equals the square of its refractive index. 



This relation, originally a purely theoretical deduction from an 

 unproved theory, has been gradually verified as our experimental 

 resources gained in power to grapple with the various difficulties 

 involved in the measurements. In the first place, the specific 

 inductive capacity is not a fixed number, but varies with the 

 nature of the electric charge, whether stationary or alternating, 



1 Abstract of two papers communicated to the Royal Society by Prof. 

 Tagadis Chunder Bose, M.A., D.Sc, Calcutta: "On the Determination 

 of the Indices of Refraction of variotis Substances for the Electric Ray," 

 and ''On the Influence of the Thickness of Air-Space on Total Reflection of 

 Electric Radiation." 



NO. 1476 VOL. 57] 



on to the delicate receiver. Unfortunately it is difficult to find 

 a substance which is as absorbent for electric radiation as lamp- 

 black is for light. Lamp-black in the case of electric radiation 

 produces copious reflection. I have tried layers of metallic 

 filings, powdered graphite, and other substances, but they all 

 fail to produce complete absorption. The only thing which 

 proved tolerably efficient for this purpose was a piece of thick 

 blotting paper or cloth soaked in an eleclrolyte. A cardboard 

 tube with an inner layer of soaked blotting paper is impervious 

 to electric radiation, and the internal reflection, though not 

 completely removed, is materially reduced. No reliance can, 

 however, be placed on this expedient, when a very sensitive 

 receiver is used. 



" After repeated trials with different forms of receiving tubes, I 

 found a form, to be described below, lo obviate many of the 

 difficulties. Instead of a continuous receiving tube, I made two 

 doubly inclined shields, and placed them one behind the other, 

 on the radial arm which carries the receiver. The first shield 

 has a tolerably large aperture, the aperture of the second being 

 somewhat smaller. The size of the aperture is determined by 

 the wave-length of radiation used for the experiment. It will 

 be seen from this arrangement, that the rays which are in the 

 direction of the radial arm, can effectively reach the receiver, 

 the slanting rays being successively reflected by the two shields. 

 With this expedient, a great improvement was effected in obtain- 

 ing a definite reading. 



" When the deviated rays are convergent, the receiver is 

 simply placed behind the shields, at the focus of the rays. But 

 when the rays are parallel, the useof an objective (placed behind 

 the first shield) gives very satisfactory results. As objectives I 

 used ordinary glass lenses ; knowing the index from my experi- 

 ments, I was able to calculate the focal distance for the electric 



