colour up to almost any variety of red*. During the present year, indeed, Piazzi Smyth has been informed by Mr. Rand Capron, of Guildford, Surrey, that the coincidence is never quite perfect, the aurora-line being slightly more refrangible than the second line of the citron band; and Piazzi Smyth is quite ready to bow before this correction from the superior spectroscope with much larger dispersion, and the evident practical ability of Mr. Capron. But the correction still leaves the reference-use of the line 5585.5, though the second of the band (which gradually goes off into invisibility), more important than the first-and just as accurate as before, if each observer will remember that a normal aurora-line reads on the comb-like natural scale of the citron band of the carbohydrogen-spectrum not 2.0, but 2.2-the wave-lengths of all the lines in that interesting band reading thus, according to various authorities :

How, therefore, Dr. Watts came in 1874 to single out the second line of the citron band for super-accurate measurement of place by mere chance, and without knowing any thing more than appears in his paper of my having used it for years as a standard reference for an almost exactly coincident cosmical phenomenon in the night sky, is a strange problem. But its due investigation I must leave untouched on now, as there are other and more world-wide important matters to be discussed in the Doctor's last paragraph, where he states with no little positiveness both what the visible spectrum concerned is chemically the spectrum of, and of what it is not.

to be seen with all ordinary and some extraordinary forms of spectroscopes in any usual and convenient formula for preparing the carbohydrogen-spectrum of a moderate intensity, I propose not to take any further account of them now, except to say that, if I ever do meet them in future spectroscopy, I shall probably call them "linelets," or some such diminutive of lines, leaving therefore our present arrangement, and also Dr. Watts's present-paper arrangement, of bands and lines intact.)

Of these last most proper lines, then (in any ordinary spectroscope with a fine slit), let us speak now touching their appearance in Dr. Watts's new-paper account of them for 1874. They are given there as though they had never been observed or measured by any one else previously; and no one is entitled to object much to that, if they are now set forth in a better and completer manner (especially for the practical use of astronomers) than in all former accounts of them. Is that, however, the case?

No! I am sorry to say; for while thirteen distinct lines are recorded, there is not the slightest indication as to their being of any but equal visibility; yet one of them is an actual Sirius for brilliance, the very brightest of all the 1st-magnitude lines or stars, and others are as faint as 15th-and 20th-magnitude stars, seldom seen by any one. Yet of all that he has actually seen, I doubt not that Dr. Watts's measured wave-length places are always respectable for accuracy, though not perfect; and I can quite enter into his statement that the best-determined of them all are the lines 5165.5 and 5585.5.

Why or how that result came about is not explained by him; I will therefore proffer two reasons of my own, which will not decrease the interest or importance of the measures.

(1) The line 5165.5 is the first of the green band, the brightest line of the whole carbohydrogen-spectrum, the one which should have been decorated with an a; and it was much enlarged on, no less than nineteen years ago, by Professor Swan for its extreme beauty, its brilliance, its definition, and its capacity for being accurately measured. And

(2) The line 5585.5, not the first, but, strange to say, the second of the citron band, was found in 1870 by Piazzi Smyth to be so almost exactly coincident with the chief, the almost only, the Angström-discovered aurora-line, that he has recommended it repeatedly since then to all aurora-observers as affording a ready and instantaneous eye-proof whether there is any variety of spectroscopic character in the chief part of the light of successive auroras-he, too, having found none in nearly thirty auroras spread through two or three years, and embracing the widest external variations from arcs of bland light to needleshaped shooting rays, and from pale yellow-green or citron

to be seen with all ordinary and some extraordinary forms of spectroscopes in any usual and convenient formula for preparing the carbohydrogen-spectrum of a moderate intensity, I propose not to take any further account of them now, except to say that, if I ever do meet them in future spectroscopy, I shall probably call them "linelets," or some such diminutive of lines, leaving therefore our present arrangement, and also Dr. Watts's present-paper arrangement, of bands and lines intact.)

Of these last most proper lines, then (in any ordinary spectroscope with a fine slit), let us speak now touching their appearance in Dr. Watts's new-paper account of them for 1874. They are given there as though they had never been observed or measured by any one else previously; and no one is entitled to object much to that, if they are now set forth in a better and completer manner (especially for the practical use of astronomers) than in all former accounts of them. Is that, however, the case?

No! I am sorry to say; for while thirteen distinct lines are recorded, there is not the slightest indication as to their being of any but equal visibility; yet one of them is an actual Sirius for brilliance, the very brightest of all the 1st-magnitude lines or stars, and others are as faint as 15th-and 20th-magnitude stars, seldom seen by any one. Yet of all that he has actually seen, I doubt not that Dr. Watts's measured wave-length places are always respectable for accuracy, though not perfect; and I can quite enter into his statement that the best-determined of them all are the lines 5165.5 and 5585.5.

Why or how that result came about is not explained by him; I will therefore proffer two reasons of my own, which will not decrease the interest or importance of the measures.

(1) The line 5165.5 is the first of the green band, the brightest line of the whole carbohydrogen-spectrum, the one which should have been decorated with an a; and it was much enlarged on, no less than nineteen years ago, by Professor Swan for its extreme beauty, its brilliance, its definition, and its capacity for being accurately measured. And

(2) The line 5585.5, not the first, but, strange to say, the second of the citron band, was found in 1870 by Piazzi Smyth to be so almost exactly coincident with the chief, the almost only, the Angström-discovered aurora-line, that he has recommended it repeatedly since then to all aurora-observers as affording a ready and instantaneous eye-proof whether there is any variety of spectroscopic character in the chief part of the light of successive auroras-he, too, having found none in nearly thirty auroras spread through two or three years, and embracing the widest external variations from arcs of bland light to needleshaped shooting rays, and from pale yellow-green or citroncolour up to almost any variety of red*. During the present year, indeed, Piazzi Smyth has been informed by Mr. Rand Capron, of Guildford, Surrey, that the coincidence is never quite perfect, the aurora-line being slightly more refrangible than the second line of the citron band; and Piazzi Smyth is quite ready to bow before this correction from the superior spectroscope with much larger dispersion, and the evident practical ability of Mr. Capron. But the correction still leaves the reference-use of the line 5585.5, though the second of the band (which gradually goes off into invisibility), more important than the first and just as accurate as before, if each observer will remember that a normal aurora-line reads on the comb-like natural scale of the citron band of the carbohydrogen-spectrum not 2.0, but 2.2-the wave-lengths of all the lines in that interesting band reading thus, according to various authorities :

How, therefore, Dr. Watts came in 1874 to single out the second line of the citron band for super-accurate measurement of place by mere chance, and without knowing any thing more than appears in his paper of my having used it for years as a standard reference for an almost exactly coincident cosmical phenomenon in the night sky, is a strange problem. But its due investigation I must leave untouched on now, as there are other and more world-wide important matters to be discussed in the Doctor's last paragraph, where he states with no little positiveness both what the visible spectrum concerned is chemically the spectrum of, and of what it is not.

*

When the red colour is decided to the eye, a red line appears in the spectrum with wave-length 6290 nearly; but the 5585 citron-line is still always in its due place, and always brighter than the red line, even in the very reddest parts of the aurora.