532 C. PIAZZI SMYTH ON THE 



the R. Soc. Edin. (vol. xxviii.) — the spectrum colours, unlike the Fraunhofer, or 

 " fixed " lines in the solar spectrum, are not fixed and unalterable in spectrum 

 place ; but are positively locomotive therein through certain limits, according 

 to the colour and the strength of the light. Hence Colour, though gloriously 

 powerful, can only be, under any method of representation, an approximate 

 indicator of exact spectrum position ; and will be most useful, when we employ 

 it on that clear understanding alone. 



Dependent then in part on what the chromo-lithographer can accomplish, 

 and what I shall be able to pay for — as the Society is not to be put to the 

 expense of colour, — I have turned to the chromatic plate in my book " Madeira 

 Spectroscopic," and have extracted thence ten well separated and easily dis- 

 tinguishable colours, — extending by equal spaces of 3000 W.N. Units on 6 plates 

 each, through all the visual spectrum depicted as here from 33,000 to 63,000 

 W. Number place. And I have also described, as well as exhibited, each of 

 those 10 colours, together with an ultra-region at the beginning and end 

 of them in a single collective plate, which I trust will be found all the title- 

 page and index, which the whole mass of the following plates requires. 



These plates are numbered, not 1 to 60, but 2 to 61 ; for the reason, that 

 the No. 1 plate after being finished in MS., was found to contain only one line, 

 and that of no very pronounced character. In the spectrum itself too, it is 

 exceedingly difficult to see, and therefore not capable of much accuracy of 

 measure, so the plate carrying it has been omitted for economy's sake. While 

 finally, at the suggestion of the Secretary R. S. Ed., I have added throughout, to 

 the two upper strips, the scale points of " Wave lengths " in modern French 

 metric terms, adopted by Angstrom in the latter years of his life, vice the 

 " inches " of his renowned and heroic Scandinavian forefathers. 



Part V. — Of Variations of Temperature and other probable 

 Sources of Minute Disturbance. 



On comparing the three Winchester Spectra carefully together, after they 

 were entered, reduced to Wave-number scale, on the final 60 sheets, — I was 

 rather disappointed to find that they did not agree more closely and minutely 

 at every point — whether as regarded (a) the exact places and appearances of 

 strong and well-known lines ; or (b) the existence or non-existence, as well as 

 the exact places, of very thin lines not hitherto generally known of, or 

 recognised amongst most observers. 



In matter (a) the anomalies were found, not only in the absolute Wave- 

 number places of the sheets of reduction, but in the original instrumental 

 records ; so that the simple intervening distances there, in three several cases 

 of certain well-identified lines between great A and great C, — measured, in 



