March 22, 1906] 



NA TURE 



495 



graphed continuously on a revolving plate, so that the 

 nature of the disappearance, whether instantaneous or 

 gradual, could be recorded. 



Numerous valuable observations of peculiar stellar spectra 

 were made during the year, including the discovery of 

 Nova Aquila? No. 2 by Mrs. Fleming. This is the eighth 

 Nova discovered by that observer from the Draper memorial 

 spectrograms. 



With the Bruce telescope 523 plates were obtained, 

 making 7504 in all, from which Miss Leavitt has discovered 

 1 129 new variable stars during the year. 



The bibliography of variable stars compiled by Miss 

 Cannon was nearly ready for publication when the Astro- 

 nomische Gesellschaft appointed a committee to undertake 

 a similar work. Prof. Pickering therefore proposes to 

 publish the Harvard work in an abridged form. 



Catalogue of 3700 Bright Stars. — A useful catalogue 

 of 3799 bright stars has just been published by M. J. 

 Bossert, of the Paris Observatory. 



This catalogue gives the magnitude and mean coordinates 

 (1900-0) of each star, and, in addition, the precession, 

 secular variation, and proper movement, together with in- 

 structions and examples for finding the star's position at 

 any given epoch. 



The stars are arranged in zones of i° of N.P.D., and 

 in each zone they are given in order of R.A., this classifi- 

 cation being considered the most convenient for meridian 

 observers. 



Stars down to the seventh magnitude are included, the 

 magnitude of Aldebaran being taken as i-o. 



Eclipse Observations at Catania. — On the occasion of 

 the total solar eclipse "1 August 30, 1905, observations of 

 prominences, by the Lockyer-Janssen method, and of the 

 variations in the terrestrial electric field were carried out, 

 during the whole clay, at the Catania Observatory. 



The results are published in No. 1, vol. xxxv., of the 

 Memorie della Societa degli Spettroscopisti Italiani, and 

 show, among other things, that the maximum effect of the 

 solar radiation corresponds to the minimum potential of 

 the atmospheric electricity. 



Micrometer Measures of Struve Double Stars. — No. 

 4078 of the Astronomische Nackrichten contains the results 

 of a series of measures of eighty-one " Struve " double 

 stars made by Dr. H. E. Lau, of the Copenhagen Uni- 

 versity Observatory. 



The position for 19000, the position-angle, the distance, 

 and the data and hour of each observation are given for 

 each star, and are followed bv brief notes bv the observer. 



SOME APPLICATIONS OF THE THEORY OF 

 ELECTRIC DISCHARGE THROUGH GASES 

 TO SPECTROSCOPY.' 



HPHE luminosity produced by an electric current passing 

 through a gas at low pressure varies greatly in 

 character, not only when we alter the nature of the dis- 

 charge, as, for example, when we pass from the arc to 

 the spark, but also in many cases at different points of 

 the same discharge. The luminosity may be of one colour 

 at one place and of a very different colour at another, and 

 spectroscopic examination shows that the spectrum of the 

 same gas often varies considerably as we proceed along the 

 line of discharge. As recent experiments have thrown a 

 considerable amount of light on the processes going on 

 in the different kinds of electrical discharge and at different 

 parts of the same discharge, the studv of the connection 

 between the changes in the electrical effects and the 

 changes in the spectra might be expected to throw some 

 light on the very interesting question of the genesis of 

 spectra. Many important points can very conveniently be 

 studied by the aid of Wchnelt's method of producing the 

 current. In this method the kathode is a strip of platinum 

 or a piece of platinum wire on which either a little lime 

 or barium oxide has been deposited. This when heated 

 to redness emits large supplies of corpuscles, and by alter- 

 ing the temperature of the platinum very large variations 

 1 on Friday, January 19, by 



in the current passing through the tube and in the potential 

 difference between the electrodes can be obtained. In our 

 experiments the current varied from a small fraction of a 

 milliampere to several amperes, and the potential difference 

 from a few volts to several hundred. 



The apparatus used is shown in Fig. 1. AB is the 

 platinum strip with the lime on it ; a thermocouple— a 

 platinum and platinum-rhodium junction — was fused to 

 this strip, and served to determine its temperature ; the 

 strip was connected with the earth, and was heated by 

 a current passing through the leads LM ; a rheostat was 

 placed in series with the heating current, and by means 

 of this the temperature could be altered gradually. The 

 anode was a platinum disc ; this was connected witli the 

 positive pole of a battery of storage cells, the negative 

 pole of which was earthed ; to allow of gradual variations 

 in the potential difference between the electrodes a 

 potential divider of 100 resistances of 10 ohms each was 

 used. The current through the tube was measured by 

 a d'Arsonval galvanometer, and the potential difference 

 between the terminals by a Weston's voltmeter. 



Some of the most interesting features of the discharge 

 are very prominent when the temperature of the platinum 

 is high', say 1400 C, and the pressure of the gas low, 

 less than 001 mm. of mercury. The discharge is light 



(h 



p' 



: delivered at the Royal Instil 

 Prof. J. J. Thomson, F.R.S. 



mo. 1899, vol 7$] 



o 



w 



blue, and its spectrum shows the mercury lines and the 

 band spectrum of nitrogen. In this case the relation 

 between the current and the potential difference is re- 

 presented by a curve like Fig. 2, the ordinates represent- 

 ing the current and the abscissae the potential difference. In 

 the case we are considering, when the wire is very hot and 

 the pressure low, the change from the dark to the luminous 

 discharge takes place very abruptly, an increase of the 

 potential difference by 1/100 of a volt being often sufficient 

 to convert a discharge where no light could be detected 

 even in a darkened room into one where the light was quite 

 bright. When luminosity appears there is a very rapid 

 increase in the current ; in some of the experiments an 

 increase in the potential difference of 1/100 of a volt 

 increased the current forty-fold. At this stage the thermo- 

 junction showed that there was no increase in the tempera- 

 ture of the platinum where the luminosity appeared ; we 

 shall see later on that it is possible by using large potential 

 differences to get such large currents through the tube that 

 the platinum becomes appreciably warmer by the passage 

 of the current. 



One point which I think very suggestive is the abrupt- 

 ness with which the luminosity round the kathode appears. 

 We see that by a very small increase in the potential 

 difference the discharge ' passes from a state in which no 

 luminosity can be detected, even in a dark room, to one 

 where the luminosity can plainly be seen in a bright light; 

 thus the molecules of the gas in the tube, just when the 

 luminous discharge is on the point of appearing, are in 

 a state in which a very small change in the electrical 



