420 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



finger ; while the fifth, in the violet, is not of the build of any of the other 

 bands ; you may say not a finger at all, but shorter, broader, sturdier, like 

 the thumb. 



Viewed in any ordinary single-prism spectroscope, each of the first four 

 of these bands begins on the red side with a strong line, followed by two, 

 three or more similar lines, but in decreasing brightness and lessening distance, 

 interspersed with haze ; while the fifth band seems to be composed of nothing 

 but haze. 



This luminous hazy mist has, however, been occasionally seen by some 

 individuals with powerful spectroscopes to be more or less resolvable into 

 faint and exceedingly close lines or linelets. And now, with my new 

 spectroscope, I have not only seen it all so resolved, but have been able to 

 measure almost every linelet by micrometer, until at last they became too 

 faint to be distinguished in any manner whatever. 



The record thus procured, proving so neatly that there is no waste, 

 neglected, unordered or accidental material in the spectrum of this cheapest of 

 all the gases, was obtained on the instrument in so very magnified a condition, 

 that the whole visible spectrum, or from red to violet on the same scale, would 

 extend to 120 feet in length. 



But this being supposed a rather longer scale than the world is as yet quite 

 ready for, though demanding far more for certain parts of the Solar Spectrum, 

 the original record has been reduced on subsequent working sheets to a 40 

 foot spectrum length. And as only the portions with very visible lines and 

 linelets for the five bands of CH in air, are given, — this particular subject 

 will yet be found in larger and fuller representation than it has probably ever 

 enjoyed before, though packed away in four only of our plates. 



The Spectrum Plates Employed here, their Methods and Symbolism. 



For the original observations and records on both the long 120 foot scale, 

 and the first reduction to the forty-foot, I am answerable myself. But the 

 drawing out of the final and finished plates, thirty-one in number for the whole 

 paper, and all save three on the same exact 40 foot Wave-number per British 

 inch scale, has been confided to Mr Thomas Heath, First Assistant in the 

 Observatory; because his handling of the pen and pencil is finer than mine; 

 and, under photo-lithographic treatment will give something like the perfection 

 of copper-plate engraving, without its superior expense. 



The said plates are intended moreover to serve more than the usual purpose 

 of such data. For though it is customary, and was actually done by Messrs 

 AlNGSTROM and Thalen in their classical case, to give both a printed list of the 

 numbers representing their micrometrical measures, and also an engraved, and 



