Title Page to the Plates of 

 MlCROMETRICAL MEASURES OF GASEOUS SPECTRA 



General Rules for the Method of Representation 

 adopted in these plates. 



(1) The Method is Negative, in that Light is represented by 

 Black, and Darkness by White. 



(2) Every straight Vertical Line, whether thick or thin, and 

 whether close to another or not, within the limits of 

 each horizontal Spectrum Strip, always stands for a 

 veritable and measured Spectroscopic Line, or monochro- 

 matic image of the Slit; and nothing else. 



(3) Lines in any other direction than Vertical, i.e., whether 

 horizontal or slanting, and from either side, or both 

 sides at once as in crossed lines, also wavy lines, — are 

 to be interpreted as Nebulous Shade only, in vertical 

 bars or bands of corresponding width at the place. 



(4) Greater or less Height or Depth, either of Lines Or Bands, — 

 is intended, in connection with the amount of ink ex- 

 pended upon them, to typify greater or less intensity 

 and visibility of such Lines or Bands. 



(5) Oones of shade arranged on a vertical central axis, indicate 



nebulous bands of pale light, shaded off towards either 



side very gradually and delicately. 



G. P. S. 



r £ del. ] A. Ritchie f Son, Photo -lith. 188* 



