in the Modern Spectroscope. 25 



to be mapped" This, as italicized by him, is very clear ; and 

 he finds such a reference in what he calls the carbon-spectrum, 

 but which others before him, whom he does not mention, have 

 called the spectrum of carbohydrogen, of acetylene, of marsh- 

 gas, and of general blue base of flame. 



Impressed next with the importance of the places of the bands 

 of that spectrum being known to great accuracy, Dr. Watts pro- 

 ceeds to give his own recent, and doubtless very good, measures 

 of the wave-lengths of certain fine lines therein, and concludes 

 with stating what substance the whole collection is the spectrum 

 of, and what it is not, both in chemistry and physics. 



The principle involved. 



Now to the principle of spectroscopic observation announced 

 in the first part of the paper I certainly cannot object, seeing 

 that it is, step by step, the very method and the very reference- 

 spectrum which I have been employing for four years past, and 

 am still using in this royal observatory, and have often pub- 

 lished upon, hoping; to promote its general adoption. My first 

 attempt indeed was not very successful ; for having sent a paper 

 on the subject to the Royal Astronomical Society on May 30, 

 1871, with the request that it might be read at the next meeting 

 and printed in the following 'Monthly Notices/ that learned body 

 (which is a very excellent body except in so far as it deals in 

 the accursed thing for all free countries, of secret committees) 

 chose to keep my paper back for six months without rendering 

 any reason why. 



However, I having met, during those months, with Professor 

 Swan of St. Andrews, who first accurately measured that spec- 

 trum's lines, so far back too as in 1856 — and I having held forth 

 to him on the delightful and universal reference for faint astro- 

 nomical spectra which I had found in that chemical spectrum 

 which he had previously made so peculiarly his own, both by 

 a priority without any question, and an accuracy which Professor 

 Kirchhoff has since then pronounced to be classical, — he was 

 induced to go to Section A of the British Association, then 

 meeting in Edinburgh, and gave them a good and sound paper 

 on the subject, besides furnishing me with his own special 

 reductions of some of his angles of the refractive indices of the 

 lines into wave-lengths, agreeably with Angstrom's normal solar 

 spectrum. 



Afterwards, too, I not only printed the so-long-stopped paper 

 in the thirteenth volume of the Astronomical Observations of the 

 Royal Observatory, Edinburgh (together with Professor Swan's 

 wave-lengths as above), but in the beginning of 1872 practised 

 the method still further on the zodiacal light at Palermo, and on 



