542 C. PIAZZI SMYTH ON THE 



single human being — is rather disturbing to private workers in science. And 

 I cannot but think the change to be a mistake, wherever at least Spectrum 

 maps are concerned, for this amongst other reasons derived from the 

 Winchester records. 



Viz., there are divers practical cases known, where the same chemical 

 element repeats a certain constellation group of lines of its own, at several 

 successive places in the length of the Spectrum. And evidently, if only for 

 recognition purposes by means of measure, it would be of advantage to every 

 one researching these matters, that the scale of a spectrum map should be 

 such, that each of these repetitions of a recondite natural phenomenon should 

 be presented of the same size, — in whatever successive colour of the spectrum 

 it may reappear. 



Now the grandest example throughout the whole Solar Spectrum, of such 

 a repetition-form, is without doubt set forth in those three most striking linear 

 constellations of great A, great B, and the Alpha band ; all of them now 

 considered to be clue to absorption by cold Ox} r gen in telluric, and super- 

 telluric, position. 



Much of the peculiar arrangement of the lines in great B and its pre- 

 liminary band, has been known for a long time past, from Angstrom and 

 Thalen* downwards. But that the arrangement in great A is exactly similar, 

 even down to the doubling of every line but one, in its preliminary band, — was 

 only discovered with the powerful assistance of one of Mr Rutherford's best 

 gratings so lately as 1878, by Professor Langley of the Allegheny Observatory, 

 U.S. ; and that a similar arrangement prevails in the Alpha band, was the very 

 recent and neat discovery of M. Cornu in Paris, in 1883. 



Whoever too has had the privilege of repeating these observations with 

 sufficient diffraction, or dispersion, power — must have been struck with the 

 extraordinary perfection of the series. In the preliminary bands for at least 

 ten couples and one single line, the emplacement is exact ; and in the subse- 

 quent line groups very nearly, though not quite, or simply so ; for they are 



* It seems by a recent publication from Upsala, that M. Thalen with prisms, saw the clear 

 duplicity of the linelets of Great B's preliminary band much better than did M. Angstrom with a 

 small grating, mounted on a theodolite stand. In fact the latter did not see them to complete identi- 

 fication ; and in his extreme anxiety not to pass beyond the modesty of observation and the truth of 

 nature, disputed long before he would allow his friend M. Thalen to draw these linelets double, as he 

 saw them without any doubt, on the manuscript for the immortal " Normal Solar Spectrum." They 

 were however so drawn at last by M. Thalen, and were so engraved by the lithographer ; but on his 

 sending a proof of his work for correction to M. Angstrom, then on his death-bed, the dying 

 philosopher, in his over conscientious desire not to exaggerate what he had really seen, toolc a pencil 

 and filled in therewith the narrow spaces between the double members of each linelet; the engraver 

 imitated the granular pencil markings ; and that is the origin of the shading by dots, quite anomalous 

 in spectroscopy, to be seen now on the finally engraved and published Atlas of the Normal Solar 

 Spectrum, in its particular plate representing that preliminary band of the great B line. 



