February i8, 1892] 



NATURE 



2>^7 



accompanied by dark lines on their more refrangible 

 sides." 



This was substantially the substance of the telegram 

 which appeared in the Standard on the following Wed- 

 nesday (February 10), with the additional remark that 

 the Harvard astronomers thought it possible that the 

 phenomena presented by the new star had been caused 

 by the collision of two celestial bodies. 



On the next day the detailed observations made on 

 Sunday night at Kensington, together with the approxi- 

 mate wave-lengths of the lines measured on the photo- 

 graphs, were sent on by Mr. Lockyer to the Royal Society. 

 From these we learn that the Nova on Sunday appeared 

 to be slightly brighter than on February 3. 



With the lo-inch refractor and Maclean spectroscope, 

 C was seen to be very brilliant, and there were four very 

 conspicuous lines in the green. Several fainter lines were 

 also seen, and a dark line was suspected in the orange. 

 Mr. Lockyer noticed that some of the lines, especially the 

 bright one near F, on the less refrangible side, appeared 

 to change rapidly in relative brightness, and this was 

 confirmed by Mr. Fowler. 



Observations of the spectrum were made by Mr. Fowler 

 with the 3-foot reflector and the Hilger 3-prism spectro- 

 scope. These call for no special remark. 



Twenty bright lines have been measured on the 

 photographs, and their wave-lengths are given in the 

 accompanying table: — 



Lines in the spectrum of Nova Aurigae. 



11= 





3933 

 3968 

 4101 

 4128 

 4172 

 4202 

 4226 

 4264 

 4291 

 4310 

 434-^ 

 4383 

 4412 

 4434 

 4469 

 4518 

 4555 

 4587 

 4625 

 4860 F 



1-^ 



o S 



I ^s ! 1^ 



I 3970 ; 

 4101 I 



4340 



4472 

 4510 

 4550 



4620 

 4860 t 



3933 1 3933 



3968 I 3968 



4101 I 4101 



4130 4130 



4172 I — 



— 4200 



— 4226 

 4268 4268 



4340 4340 



- 4383 



4415 1 4410 



4472 I 4472 



4860 I 4860 



The table also shows probable coincidences with the 

 lines in the spectra of the Wolf-Rayet stars as photo- 

 graphed by Prof. Pickering, dark lines in Orion stars 

 photographed at Kensington, and bright lines in the 

 Orion nebula photographed at Mr. Lockyer's observa- 

 tory at Westgate. 



In addition to the lines recorded in the table, the 

 photographs in the spectrum of the Nova show several 

 lines more refrangible than K. They probably include 

 some of the ultra-violet hydrogen lines. 



All the lines in the spectrum of the Nova are broad, 

 although in a photograph of the spectrum of Arcturus, 

 taken with the same instrumental conditions, the lines 

 were perfectly sharp. It is also important to note that 

 the broadening of the lineS^ is not accompanied by any 



NO. I 164, VOL. 45] 



falling off of intensity at the edges, as in the case of the 

 hydrogen lines in such a star as Sirius. With the method 

 employed in taking the photographs, long exposures are 

 liable to result in a thickening of all the Imes on account 

 of atmospheric tremors. The lines would also be thick if 

 the Nova be hazy. In the photograph, however, all the 

 lines are not equally thick. 



If the lines are similarly broadened when a slit spectro- 

 scope is employed, the effect must be due to internal 

 agitations, for if different regions of the Nova are moving 

 with varying velocity, or with the same velocity in different 

 directions, a normally fine line might be widened in the 

 manner observed in the photographs. 



With regard to the bright and dark lines the paper 

 states as follows : — 



" A somewhat similar phenomenon has already been 

 recorded by Prof. Pickering in the case of fi Lyra^, and 

 this has been confirmed by a series of photographs taken 

 at Kensington. In this case the bright lines are alter- 

 nately more or less refrangible than the dark ones, with 

 a period probably corresponding to the known period of 

 variation in the light of the star. The maximum rela- 

 tive velocity indicated is stated by Prof. Pickering as 

 approximately 300 English miles per second. 



" In the case of Nova Aurigae, the dark lines in all four 

 photographs taken at Kensington are more refrangible 

 than the bright ones, so that as y^t there is no evidence 

 of revolution. 



" The relative velocity indicated by the displacement of 

 the dark lines with respect to the bright ones appears to 

 be over rather than under 500 miles per second. The 

 reduction is not yet complete. 



" Should the photographs which may be obtained in the 

 future continue to show the dark lines displaced to the 

 more refrangible side of the bright ones, it will be a valu- 

 able confirmation of my hypothesis as to the causes 

 which produce a new star — namely, the collision of two 

 meteor-swarms On this supposition the spectrum of 

 Nova Aurigae would suggest that a moderately dense 

 swarm is now moving towards the earth with a great 

 velocity and is disturbed by a sparser one which is 

 receding. The great agitations set up in the dense swarm 

 would produce the dark-line spectrum, while the sparser 

 swarm would give the bright lines" 



ELECTRODYNAMIC THEORIES AND THE 

 ELECTROMAGNETIC THEORY OF LIGHT} 



T N a former article we endeavoured to give an account 

 •*• of the first part of M. Poincare's " Electricitd et 

 Optique," in which he dealt with the electric and 

 magnetic theories expounded in Maxwell's treatise. In 

 Part II. he now compares the theory of electromagnetic 

 action given by Maxwell with the somewhat more general 

 theory put forward by Helmholtz in his celebrated paper 

 on the equations of motion of electricity {Po^a^g. Attn., 

 cii. p. 529, or Wissensch. Abhand., vol. i.) ; discusses the 

 condition which must hold in order that the two theories 

 may coincide ; and, after a masterly exposition of the 

 various consequences which flow from Maxwell's theory, 

 finishes with a very valuable analysis of the theoretical 

 and experimental work of Hertz. 



In the first chapter M. Poincard deals with the formula 

 of Ampere for the mutual action of two current elements. 

 The method adopted is founded on the following three 

 principles assumed from Ampere's experiments: — 



(l) That a current in a conductor may be replaced by 

 an equal current in a sinuous conductor nowhere 

 deviating from the first by a distance comparable with 

 the distance of the latter from any element of the other 

 conductor acted upon. 



I " filectricite et Optique." II. Les Theories de Helmhnltx et les Ex- 

 periences de Heitz. ParH. Poincare', Membredel'Institut. (Paris : Georges 

 Carr^, 1891.) 



