SIR. WILLIAM HUGGINS— CAMPBELL. 315 



On a subsequent evening, June 25, I repeated these comparisons, when the 

 former observations were fully confirmed in every particular. On this evening 

 I compared the brightest band with that of carbon in the larger spectroscope, 

 which gives a dispersion of about five prisms. 



The remarlvably close resemblance of the spectrum of the comet to the spec- 

 trum of carbon necessarily suggests the identity of the substances by which in 

 both cases the light was emitted. 



The application of the Doppler-Fizeau principle to the measure- 

 ment of stellar velocities has assumed great importance in astronomi- 

 cal investigation. It is now easy to look backward and say that this 

 importance was inevitable, but it was not easy, half a century ago, 

 to look forward and say that this must be so. It is characteristic 

 of the pioneers in this field that they were slow to publish their ideas 

 and observations. 



It was Fizeau, in 1848, who first enunciated the principle correctly 

 that motions of approach and recession must cause corresponding 

 shiftings of the entire spectrum, including the dark lines of Fraun- 

 hofer, toward the violet and red, respectively, but without change of 

 color. Fizeau also outlined methods for applying the principle to 

 measuring the motions of celestial bodies toward and away from the 

 observer. While these methods were sound theoretically, they were 

 unpractical. All matters spectroscopic were then mj^sterious, and 

 Fizeau's statements attracted no serious attention. In fact, his lec- 

 ture on the subject in 1848, before a minor society in Paris, was not 

 published until 1869. In the meantime the subject was receiving at- 

 tention on the theoretical and laboratory sides from Fizeau and 

 Clerk Maxwell, and on the stellar side from Huggins and Miller, and 

 from Secchi. Secchi's paper in Comptes Rendus, Paris Academy, 

 dated March 2, 1868, describes his search for high velocities of the 

 stars in the line of sight, conducted under encouragement from 

 Fizeau, which led to merely negative conclusions; and he remarked 

 that success in detecting velocities in the line of sight no greater than 

 that of the earth in its orbit would require instrumental equipment 

 more powerful than was then at his disposal. 



Almost simultaneously appeared a paper by Huggins and Miller 

 in the Philosophical Transactions, dated April 23, 1868, from which 

 the following paragraph is quoted : 



In a paper "On the spectra of some of the fixed stars " by myself and Dr. 

 W. A. Miller, treasurer Royal Society, we gave an account of the method by 

 which we had succeeded during the years 1862 and 1863 in making trustworthy 

 simultaneous comparisons of the bright lines of terrestrial substances with 

 the dark lines in the spectra of some of the fixed stars. We were' at the time 

 fully aware that these direct comparisons were not only of value for the more 

 immediate purpose for which they had been undertaken, namely, to obtain in- 

 formation of the chemical constitution of the investing atmospheres of the 

 stars, but that they might also possibly serve to tell us something of the motions 



