528 C. PIAZZI SMYTH ON THE 



These 6000 fiducial steps had, of course, next to be transferred to sheets 

 representing a Wave Number Scale of equal parts ; and in order that the 

 ultimate comparison of different authorities might be as instant as possible, 

 these latter sheets must be of a collective character and contain all the 

 authorities, at each point, one under the other. 



To this end, 60 plate forms of a long or double quarto size were prepared, 

 each carrying 500 units of Wave-number scale, as adapted to a spectrum now 

 of only 80 feet in length, and in strips one under the other five times repeated. 

 In the three lower strips the three Winchester spectra were entered by 

 measure ; and in the two upper, the standard tests and critical references. 



The topmost of these two reference strips, excepting for the three first 

 sheets of the series, wherein Kirchoff's celebrated spectrum was followed, could 

 not but be devoted to Angstrom's Normal solar Spectrum ; not only on account 

 of the accuracy of his absolute places, and his long priority — but for his, and 

 his friend M. Thalen's invaluable chemical equivalents of solar lines ; and as 

 they were now stretched out on these new sheets to about ten times the length 

 they occupied in the original Upsala Atlas, — I was much pleased to find that a 

 certain amount of microscopic confusion which has been complained of by some 

 persons there, was completely corrected. 



The second however of the reference strips was confined both to solar lines 

 and to the most advanced maps of them by any of the later observers, as 

 Professor Vogel, M. Fievez, M. Cornu, M. Thollon, Professor Young of 

 Princeton, and Professor Kowland, Baltimore, U.S., — duly naming each of 

 those great authorities whenever cited. 



But it is now time that I should render due thanks, and give proper praise 

 to Mr Thomas Heath, First Assist. Astron. R. Obs. Edin., for the very great 

 help he has afforded me in this section of the work. For not only has he, in 

 the long conical sheets of the three Winchester spectra, inserted the nine sub- 

 dividing lines and their numbers between each pair of my 600 preliminary 

 points thrice repeated ; but he has had the whole responsibility of copying, 

 introducing, and greatly enlarging at the same time Angstrom's celebrated 

 Normal Solar Spectrum with all its chemical references in clearer guise, into 

 the topmost horizontal strip of each of the final and collective plates ; — besides 

 doing the same for all the consecutive selections of various spectra introduced 

 into the second reference strip, — where too the various depths of lithographic 

 tints employed by some of the authorities, could not have been reproduced 

 certainly and satisfactorily in printable form, except by such admirable and 

 clear-lined penmanship as Mr Heath's. 



