238 



KNOWLEDGE. 



June, 1913. 



the prismatic camera ; by Messrs. Frost and Park- 

 hurst, at Yerkes ; by Dr. Curtis and others, at the 

 Lick Observatory. Most comets hitherto observed 

 have given continuous spectra, due to reflected sun- 

 light, and a number of bright lines or bands, whose 

 nature varies from one comet to another and whose 

 identification has been a source of some controversy. 

 Most commonly these bands have been identified as 

 " hydro - carbon," 

 since comparisons 

 of cometary spectra 

 with the spectrum 

 of defiant gas and 

 other hydro - car- 

 bons gave a very 

 close correspon- 

 dence in position 

 and appearance. 

 There is also a 

 banded spectrum 

 due to carbon mon- 

 oxide, and another 

 identified with cya- 

 nogen. The earlier 

 observations of 

 Pluvineland Baldet 

 indicated the pre- 

 sence of cyanogen, 

 and the absence of 

 continuous spec- 

 trum, but others 

 arrived at contra- 

 dictory results. 

 The presence of 

 carbon monoxide, 

 the doublets, twelve 



of which onlv were detected by Professor Fowler in 

 the laboratory spectrum of this substance, was in- 

 dicated by twenty-one in Pluvinel and Baldet's 

 results, and these bands can be arranged into a series, 

 as pointed out by Professor Fowler, in a paper in The 

 Astrophy steal Journal. Additional demonstration of 

 their results is obtained from photographs taken by 

 Dr. Curtis, of the Lick Observatory (Figure 245), the 

 carbon monoxide spectrum being placed below that 

 of the comet for easy comparison. Out of the five 

 or six recent comets which have shown the low- 



kind permission 



Figure 245. 



of Prvfissor A. Fezvlei, J-'.K.S. 



Comet Morehouse (1908 c) March 20th, 1909. 



Upper part of figure, Objective spectrogram 7 h H. D. Curtis, Santiago, 



Chile. Lower part of figure, spectrogram of Carbon monoxide, pressure 



0-01 mm. A. Fowler, South Kensington. 



pressure carbon monoxide spectrum, it was only in 

 this comet that the bands were bright in the head 

 as well as in the tail. Observations by Campbell 

 and Albrecht, at Lick, showed the " presence of 

 carbon and cyanogen, though the" second cyanogen 

 band was apparently absent. New radiations 

 suggested as due to nitrogen and other substances, 

 also, were suspected from some photographs. 



Professor Newall 

 has suggested that 

 the apparent simi- 

 larity in the spectra 

 of many comets 

 is not so much due 

 to a similarity in 

 the materials of 

 which the} - are 

 composed, as to 

 the fact that these 

 " hvdro - carbons, 

 nitro-carbons, &c," 

 are present in the 

 regions near the 

 sun through which 

 the comets pass 

 and are " rendered 

 incandescent by 

 some processes' 

 connected with the 

 motion of the solid 

 parts (including 

 dust) of the head 

 of the comet 

 through the va- 

 pours, or with the 

 emission of some 

 influence from the comet head." 



In comets, as has been suggested by more than 

 one writer, we seem to have the development of light 

 without sensible heat, phenomena of luminescence ; 

 the repulsion of their tails by the sun is attributed to 

 the agency of light-pressure, though other theories 

 have been invoked also. The action of the resisting 

 medium upon the orbits of these bodies, and possibly 

 upon the materials of their tails, is another point 

 about which we are gradually gaining additional 

 information. 



CORRESPONDENCE. 



CONSIDERATIONS ON THE PHYSICAL 



APPEARANCE OF THE PLANET MARS. 



To the Editors of " Knowledge." 



Sirs, — The article by Mr. E. M. Antoniadi on the above 

 subject in your last issue contains statements which I think 

 should not pass without comment. The interesting questions 

 arising from the markings on the planet Mars and the true 

 nature of the markings themselves are by no means so con- 

 clusively settled as is assumed by Mr. Antoniadi in his 

 article. 



He has stated only one side of the case, and to my mind the 

 arguments of the advocates of what Mr. Antoniadi calls " the 



canal myth " have not been properly stated or answered. It 

 is, I think, well that readers of " Knowledge," some of 

 whom may have had no previous acquaintance with the 

 subject, should know that the matters contained in Mr. 

 Antoniadi's article are not established truths. 



I have never before seen it stated that " canals " appear 

 straight and not curved at the edge of the disc. It has been 

 my own experience that no markings, of the nature of canals 

 or otherwise, can be seen near the limb of Mars, owing, 

 obviously, to the planet's atmosphere ; and I had previously 

 understood that this experience was universal. It is stated in 

 the article that Mr. Denning could see, with his ten-inch 

 reflector, the true nature of the Martian canals. Is it not 



