4 ATLAS OF ABSORPTION SPECTRA. 



selective absorption, and it tells even less about substances presenting 

 weak, general absorption. Another important respect in which Formanek's 

 diagrams fail to give the data required by the first sentence of this section is 

 that he confined his measurements to eye observations, unaided by phos- 

 phorescent screens, and hence he omitted the entire ultra-violet region. 

 In fact, his wave-lengths have the limits 420/j', and 741,"/^, i. e. , from 

 "above" the G line to a little " below " the a line. Formanek used a prism 

 spectroscope to the dispersion of which he gives no clue. 



To fill in this gap in the then existing collections of absorption spectra 

 the present research was begun in the spring of 1903. Its chief object is to 

 furnish graphical representations, on a normal scale of wave-lengths, of the 

 absorption spectra, both in the visible and in the 7(ltra-violet regions, of a reasoft- 

 ably large number of compounds. 



The most obvious use to which such a collection can be put is the pro- 

 duction of color screens either for photographic work or for removing higher 

 orders of spectra from the first order, in the case of diffraction gratings. It 

 also makes possible the selection of such solutions as will transmit relatively 

 narrow, and hence roughl}^ monochromatic, regions of the spectrum. Such 

 solutions are often convenient substitutes for somewhat elaborate pieces of 

 apparatus which first disperse the light by a prism (or grating) and then 

 permit any desired portion of the resulting spectrum alone to continue unin- 

 terrupted by means of a suitable slit and screens. Other directions in which 

 the data given below may be of practical value need not be pointed out here. 



SELECTION OF MATERIAL, APPARATUS, ETC. 



That a great deal of time was consumed in constructing apparatus and 

 in performing preliminar}' experiments is shown by the fact that, although 

 the investigation was entered upon in the spring of 1903, it was not until 

 July, 1904, that the first really satisfactory negative was obtained. Only 

 aqueous solutions of the aniline dyes have been investigated up to the present 

 time. As is well known, the position of an absorption band may be shifted 

 within wide limits by varying the solvent ; * moreover, many aniline dyes 

 are insoluble, or nearly so, in water. On this account it would have been 

 desirable to have made use of the alcohols, benzol, and other organic com- 

 pounds as solvents for the media under investigation. But difficulties were 

 met with which were not overcome until the stud}' of the dyes was completed. 

 Chief among them may be mentioned the rapid evaporation of the fluid held 

 between the quartz plates. Attempts were made to obviate the difficulty by 

 painting the edges of the wedge with melted paraffin, but the heat of the 

 spark was sufficient to drive off the greater part of the fluid before the 



*See Nos, 158 and 165. 



