VISUAL, GRATING AND GLASS-LENS, SOLAR SPECTRUM. 531 



confounded with any genuine Fraunhofer, or spectroscopic line proper, — and 

 have called attention to the fact, and the principles on which it is based, at the 

 foot of every one of the 60 plates now presented. 



These plates are further, though only half the size * of the original records, 

 yet still on so very large a scale, that the places of any lines thereon, despite 

 much roughness in the drawing, may yet be read off to such an accuracy, as 

 not to require any columns of printed numbers to follow. There only remains 

 therefore the propriety, when the plates are so numerous, to devise some easy 

 and effective Index to them and their chief contents. 



Part IV. — Indexing by Colour. 



The want of something of that kind becomes most evident, when some one 

 line has to be looked for among actual thousands of others, without its exact 

 Wave-Number place being perhaps known beforehand ; and not known most 

 probably on account of the Scientist or Student having been accustomed to use 

 some other Spectrum scale, as either French Wave Lengths, or Kirchoff's 

 private Prism numbers, or the devices of some optician. 



But no matter what strange, artificial and human devised spectrum scale any 

 one may have been using, — he must also, if an observer, have had Nature's 

 inimitably beautiful, and effective general indexing of spectrum place by Colour, 

 before his eyes again and again ; till those colours must, if he has a soul, have 

 been impressed involuntarily and indelibly, on the tablets of his heart in thankful 

 admiration of God's glorious Creation. Wherefore such a person's search for 

 any particular lines, guided otherwise by merely the one remaining, self- 

 evident feature, viz., their configuration in black and white, — a configuration 

 which may repeat itself very nearly, and therefore deceptively, many times in 

 the course of the whole spectrum, — will be enormously aided, expedited and 

 rendered more agreeable too, if, knowing beforehand that the group he wants 

 is in the Green, — he finds 6 out of the 60 sheets coloured Green, and the rest 

 of them steeped in colours as easily distinguishable from Green as they can 

 well be. 



The manner however of introducing this colour into the Winchester work, 

 is again partly symbolic ; in so far as, instead of one colour blending insensibly 

 into another, each is inserted in a flat tint, perfectly uniform from beginning to 

 end ; but for that very reason, by so much the more easy to be separated by the 

 eye, from either the preceding or the following colour. Nor need this be 

 considered much of a violation of the more important laws of the Natural 

 Spectrum ; because, as I have shown several years ago in the Transactions of 



* This note of size refers to the drawings for the plates. The actual prints, for economy's sake, are 

 only one-third the size. 



