THE SPECTROSCOPE AND THE WEATHER. 243 



in late years been increased to a most astonishing extent. The ques- 

 tion is important, and somewhat new as well. I propose, therefore, 

 to devote the remainder of my space to its answer, rather than to the 

 practical rules for using the smaller instruments, especially, too, as 

 they have been already introduced to the public, both by my friend 

 Mr. Rand Capron, in his pamphlet " A Plea for the Rain-band," and 

 by myself, in the fourteenth volume of the " Edinburgh Astronomical 

 Observations"- also in the "Journal of the Scottish Meteorological 

 Society," and in the September number of the " Astronomical Regis- 

 ter for 1877." 



The greater part of higher power spectroscopes are not suitable to 

 rain-band work, for their fields are usually too dark. But having 

 recently built up for myself a large-sized variety of the instrument, 

 possessing perhaps the greatest combination of power with transpar- 

 ency yet attained, and having it always mounted in an upper chamber 

 looking out at an altitude of about 5 over the northwestern horizon 

 (or most suitably for rain-band work), I will try to describe shortly its 

 action therein. 



The classical " rain-band," which in the little instrument is merely 

 a very narrow fringe to an almost infinitely thin black line, is so mag- 

 nified laterally in the larger instrument as to fill the whole breadth 

 of the field. The thin black line before spoken of is now not only 

 split into two, but these are both strong, thick, sharply defined lines, 

 separated from each other by six or seven times the breadth of either. 

 These are the celebrated solar D-lines, Dl and D2, arising from the 

 sodium metalloid burning or incandescent in the sun. They are, 

 therefore, perfectly uninfluenced by changes of the terrestrial atmos- 

 phere, hot or cold, wet or dry, and are, therefore, invaluable as ref- 

 erences for degree of visibility of the water-vapor lines and bands 

 which rise or fall in intensity precisely with those changes. There are 

 several of these earthly water-vapor lines and bands in and between 

 and about the D-lines themselves ; then a long breadth of band to- 

 ward the red side of Dl; then a pair of lines not so widely apart as 

 the D-lines, but sometimes just as sharp and black; then two or three 

 fainter bands ; then a grand triple, of which the nearer line some- 

 times attains greater blackness than either D-line; then beyond that 

 three distinct, equal-spaced, isolated bands ; and, farther away toward 

 the red, a stretch of faint haze and haze-bands. 



All these go to make up the one thin rain-band of the little spec- 

 troscopes ; and I fortunately had, through the month of August and 

 the early days of September, occupied myself each morning in noting 

 the greater or less intensity of each and all these water- vapor lines and 

 bands in terms of the two solar constants Dl and D2 ; and every such 

 morning there was an abundance of details to see, to recognize, and 

 to measure. But on the morning of Monday, September 4th, when the 

 little instrument had truly enough marked on its very small scale, 



