428 



NATURE 



[August 30. 1900 



patch which it produces even across a sheet of metal on a 

 screen of barium platino-cyanide, the instantaneous discharge of 

 an electrified body brought near to the substance, and the 

 sparks passing when radium is brought within a few centimetres 

 of the spark gap. The magnetic deflection of the rays could, of 

 course, not be made evident to such a large audience. But the 

 original negatives could be projected, and they showed the 

 curvilinear propagation of the rays in a magnetic field. 



The new bodies constantly project matter endowed with a 

 great velocity. Neighbouring bodies are impregnated with it, 

 and become radio-active in turn. These particles attach them- 

 selves, not only to objects, but to persons as well, so that M. 

 Curie will be condemned for some time to abandon every kind 

 of electrostatic research. No electrometer remains charged in 

 his neighbourhood, and it is certain that if radium had only 

 been as plentiful as gold, static electricity would never have 

 been discovered. 



In the same domain, important generalisations have been 

 made, such as the theory of dispersion in metals, founded by 

 M. Drude upon the electron theory, of which the author gave 

 an account to the section. 



The sixth section, under the presidency of M. Mascart, 

 occupied itself with cosmical physics. Terrestrial magnetism 

 should undoubtedly have formed part of the work of this 

 section, but the Meteorological Congress which will shortly meet 

 intends to make that the principal object of its studies, and it 

 was evidently necessary to leave it aside. 



Yet the work of this section was very fruitful. Here, 

 naturally, observation still holds a predominant place, as in the 

 work of the Swiss physicists, with M. Hagenbach at their head, 

 on glaciers ; and the detailed study of oscillations of lakes by 

 MM. Sarasin and Forel, who brought their results before the 

 Congress. 



In the department of atmospheric electricity, a very good 

 account was given by M. F. Exner, and Mr. Paulsen gave an 

 account of the Danish expedition to Iceland for the study of the 

 aurora. The evaluation of the solar constant by M. Crova, ac- 

 cording to recent researches, and a very ingenious theory of 

 suns-pots established by M. Birkeland after troublesome calcula- 

 tions, were heard with much interest. Finally, M. C. Dufour 

 showed how, without the help of any laboratory apparatus, the 

 approximate brightness of the stars could be determined. 



It had seemed useful to collect in a seventh section some 

 works relating to biology. In the absence of M. d'Arsonval, 

 this section, presided over by M. Charpentier, did a great deal 

 of good work, and justified the idea of the organisers of the 

 Congress. The application of physical and mathematical 

 methods to the transmission of energy in organisms, to which M. 

 Broca has devoted attention for a considerable time, and the 

 curious retina phenomena studied by M. Charpentier, gave this 

 section a vast field of discussion. Finally, the new theory of 

 accommodation established by M. Tscherning received the sanc- 

 tion of a very largely attended meeting, while M. Henocque 

 spoke of the spectroscopic methods used in biology. 



The proceedings of the Congress were not confined to sec- 

 tional work and general ^meetings. A visit to the laboratories 

 of the Sorbonne and the Ecole Polytechnique showed many ex- 

 periments in progress, installed by the professors of these 

 establishments or their provincial and foreign colleagues. These 

 could only be properly appreciated by observing them closely 

 and in small groups. 



Shall I speak of the reception in the Jardin de I'Elysee, 

 whither the President of the Republic invited several Con- 

 gresses to witness a theatrical performance ? Or of the charm- 

 ing soiree for which Prince Roland Bonaparte had placed at 

 the disposal of the organisers his vast and magnificent library for 

 a number of interesting experiments ? This soiree, which will 

 leave in the minds of all who were present the most agreeable 

 memories, would itself deserve a lengthy description. But I 

 cannot conclude this already lengthy article without saying how 

 much the French physicists have been touched by the sympa- 

 thetic action of the foreign secretaries of the Congress, who 

 deposited a magnificent crown on the modest tomb of the great 

 Fresnel, of which the Societe Fran9aise de Physique has con- 

 stituted itself the guardian. A moving speech by M. Warburg, 

 and a warm expression of thanks by M. Cornu, president both 

 of the Congress and of the Society, referring in a few words to 

 the life of that great physicist, ended this first Congress, where 

 so many new thoughts have been born, and so many friendships 

 made or consolidated. Ch. Ed. Guillaume. 



NO. 1609, VOL. 62] 



ORIENTATION OF THE FIELD OF VIEW OF 

 THE SIDEROSTAT AND COELOSTAT. 



/"OBSERVERS who have practical acquaintance with the 

 ^^ siderostat and heliostat are familiar with the fact that 

 while the reflected image of a star may be kept stationary, the 

 images of surrounding stars have a rotation around it ; while if 

 the sun is the object viewed in the mirror, the image will rotate 

 about the axial ray. It is on account of this rotation of the field 

 that neither the siderostat nor the heliostat can be used with a 

 fixed telescope for celestial photography, except for objects 

 which can be photographed with short exposures. 



Certalin unexpected peculiarities of this motion have recently 

 led Prof. Cornu to investigate the general laws governing the 

 rotation of the field in both instruments {Comptes rendus, vol. 

 cxxx. No. 9, 1900 ; Bullelin Astronomique, February 1900). 

 Some of the results at which he has arrived are of great interest, 

 and we believe attention has not been previously drawn to 

 them, although they could have doubtless been derived from 

 Orbinsky's formula for the orientation of the field (" Die totale 

 Sonnenfinsternisse am 9 Aug. 1896"), or from other formulae 

 which have been employed by observers as occasion required. 



Prof. Cornu first discusses the general question of the orienta- 

 tion of the field, irrespective of the mechanical means of 

 retaining the reflected image in a fixed position. In Fig. i, 

 NESW represents the horizon, z the zenith, p the pole, pd the 

 hour circle of the star u, and d' the point of the horizon towards 

 which the rays are reflected, pn is equal to the latitude of the 



Fig. 



-Orientation of field of siderostat. 



cos p = cos a cos L ; tan « 



place, = L ; pd is the polar distance of the star, = 5 ; spd is 

 the hour angle of the star, •= h. For the purposes of calculation 

 the point d' is defined by its polar distance pd' = p, and by the 

 angle spd' = a> which the plane pd' makes with the meridian ; p 

 and £0 can be determined in terms of the azimuth of d' ( = SD' = a, 

 reckoned positive towards the west) and the latitude, by solving 

 the right-angled triangle psd', in which PS = 180° -L ; thus 



tan g 

 sin L" 



The normal to the mirror must always bisect the arc dd' of a 

 great circle, at M, so that the position of the reflected ray from 

 any part of the sphere can be easily determined. Thus the 

 image of p is at p' in the continuation of the hour circle PM, MP' 

 being equal to PM. To determine the orientation of the field, it 

 is most convenient to ascertain the direction, after reflection, of 

 the point p, since it is a fixed point on the sphere. Taking the 

 plane of pd'Pq as the reference plane, and its trace on the sphere 

 as a fixed direction, the orientation of the reflected pole is con- 

 veniently defined by the angle PqD'p' =: Y, which can be readily 

 calculated, as also d'p', the distance of the reflected pole from the 

 centre of the field. 



Applying this in the first place to the siderostat, where the 

 reflected rays are south or nearly so, and the angle a conse- 

 quently small. Prof. Cornu obtains the following results : — 



(i) The reflected image of the pole describes a circle round 

 the centre of the field, with a radius equal to the polar distance 

 of the star observed. 



