VISUAL, GRATING AND GLASS-LENS, SOLAR SPECTRUM. 529 



Part III. — Graphical representation in place of printed 



NUMBERS ALONE. 



The final step, that of transferring into the three lower strips of each of the 

 60 plate forms above alluded to, every line of each of the Winchester Spectra 

 from the long sheets with the conical scales — -had of course to be carried out 

 rigidly by myself; and as it has also been done on a partly symbolic plan of 

 my own, intended to secure greater trustworthiness in spectrum drawings for 

 the future, — I may as well say a few words upon it at this point. 



For the middle of the Spectrum, the ordinary method of representing all 

 the stronger Fraunhofer lines, as vertical and parallel black lines ruled of more 

 or less thickness, but of equal height on a horizontal strip of white paper, — 

 such a method, I say, is just about as good a one as can be desired ; for 

 excepting colour, which has been hitherto tabooed in all high class Solar 

 Spectrum Maps, the white paper stands expressively enough for the brilliant 

 continuous part of the Spectrum of the Sun, and black ink lines also well 

 represent the darkness of sharply-defined Fraunhofer lines seen thereon. 



But when exactly the same method is carried out to each end of the 

 Spectrum, where of course the continuous spectral light of the hitherto 

 luminous back-ground in Nature at last vanishes in darkness, and no black 

 lines can be seen clearly, in a quite, or even an almost, dark field, — such 

 method I would beg to point out, misleads those who use such maps, 

 grievously. For they are led to believe that there is just as much continuous 

 spectrum light between, or as it almost looks behind, the lines of the pre- 

 liminary band of Great A, as there is between those of Great B, or of the 

 Alpha band ; and such reading students may form very erroneous estimates of 

 the probable error attending on the observations of place for the first named 

 lines, or on the distribution of lines, their thickness and the degrees of definition 

 prevailing among them. 



To meet this imperfection in previous maps, I have introduced into mine, 

 towards each end of the spectrum, a black shade running along the lower side 

 of the otherwise white horizontal strip, and gradually rising in it as the 

 spectral light fails ; until, when that ceases any longer to be visible, the black 

 shade has risen to the top of the horizontal white strip, and eclipses it from 

 that place onwards. Hence at any intervening point between the full height 

 of the white strip near the middle of the spectrum, and its final extinction at 

 either end, readers may judge of the degradation of the light, by the com- 

 parative heights of the black shade below and the white paper above it ; or 

 they may imagine the sort of gray that would be produced over the full height 

 of the strip, by smearing upwards, though but approximately, the amount of 

 black contained in that part which is so coloured by the shade below. 



