
IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 781 
The observations made on the above varieties of glass, and on several thick- 
nesses of each, followed by a few of the more immediate conclusions for prac- 
tical use derivable from them, will be found,— 
First, in the printed Appendix I., pp. 805-815 ; 
Second, in graphical form in Plates Nos. 1 to 3 MS., and Plate 1, engraved. 
In these Plates, while the light which has been transmitted is indicated by 
a white wave depending from a level line, the light which has been stopped is 
shown by a differential dark area, between the white wave of transmitted 
light, and the outline (similarly depending) of the whole wave of light which 
had previously offered itself to be transmitted from the continuous spectrum of 
the bright reference flame employed. 
This duplex wave method was adopted on considering that,—when light has 
been stopped out from any part of the length of an absorption spectrum, no 
conclusions can safely be formed therefrom, either as to the energy of the 
stopping power of the interposed and intercepting medium, or as to the 
accuracy of which the observation was capable—unless we know how much, or 
how little, light there originally was at that part to be stopped; the quantity 
moreover varying exceedingly in the course of any spectrum. 
_ The particular projection adopted for all these refraction spectra is one 
which, while giving a scale of equal parts on the paper to “ Number of Light 
waves in an Inch, British ;” and in figures increasing most naturally in the same 
direction as the increase of refrangibility,—has the further advantage in practice 
of somewhat unlocking the contents of the closely compressed-up red-end of 
the spectrum, as given more or less by all prisms,—and yet not stretching that 
end out, nor oppositely compressing the violet end, to the needlessly extra- 
vagant degree for useful spectrum details, of all difraction spectra.* 
Our projection in fact represents nearly a mean between prismatic, and 
“ orating,” spectra; while its scale is as absolute and general as any other, and 
very much more agreeable to common sense and use than those which go 
against refrangibility, the very parent of any and all spectra by prisms. It is of 
course far from being absolutely new; though it has never beenemployed on a 
large scale ; and has been too much lost sight of in recent years, when either 
arbitrary instrumental scales, or equal-part wave-length almost caricatures, 
* M. Angstrom’s grand Normal Solar Spectrum Map is a type of diffraction projections. ‘“ Wave- 
lengths ” form there a scale of equal parts along the whole of its length of 137 inches nearly. For one 
purpose of theory that wave-length scale may be convenient; but evidently Nature declines to be 
bound by it in other respects; for in the first 30 inches of it, there are only 98 solar lines; and in the 
last 30,so many as 522; so that they hardly have standing room. 
An ingenious idea was proposed some years ago by Prof. Alexander S. Herschel, for employing 
both a nature-founded scale, and in equal parts on the paper for prismatic spectra almost as observed ; 
or with all their exaggerated length of the violet end. The method consisted in employing, not wave- 
numbers per inch, but their squares; and it is nearly true for the average of prisms, But its numbers 
are impracticably large, and it does too little justice to diffraction spectra, 
