332 F. B. Pitcher — Absorption Spectra of Blue Solutions 



Aet. XXX Y. — On the Absorption Spectra of certain Blue 

 Solutions. (From Spectro-photometric Determinations by 

 Mr. F. B. Pitchee.) 



[Contributions from the Physical Laboratory of Cornell University. Communi- 

 cated by Edward L. Nichols.] 



II. 



Blues and violets obtained by absorption in pigments and 

 solutions, differ in several respects from those colors which 

 approximate in hue to the longer wave-lengths of the spec- 

 trum. They are much less completely saturated, as a rule, 

 and they show irregularities of composition, not commonly 

 met with in absorption reds and yellows. 



It is an easy matter, for instance, to find reds the absorption 

 spectra of which show no rays lying beyond the D line, but 

 blue solutions transmit all the wave-lengths of the visible 

 spectrum in considerable quantity ; so that an attempt to iso- 

 late a pure absorption blue, by increasing the density of the 

 solution or the thickness of the absorbing layer, or by dimin- 

 ishing the intensity of the light traversing the latter, results in 

 the extinction of the shorter wave-lengths along with those 

 lying in the green, yellow and red. It is moreover a matter of 

 common knowledge among spectroscopists that the absorption 

 spectra of very many blue substances, possess a more or less 

 prominent red band, with indications frequently of selective 

 absorption in other regions. 



In view of the fact that precise knowledge of the distribu- 

 tion of energy in the visible spectra of this class was almost 

 entirely lacking, our acquaintance with them being for the 

 most part confined to what may be learned by inspection, it 

 seemed of interest to submit the solutions of certain charac- 

 teristic substances of the class in question, to a spectro-photo- 

 metric analysis. This has been done, under the writer's direc- 

 tion, by Mr. F. B. Pitcher, from whose Thesis upon this subject 

 the measurements to be described in this paper have been taken. 



Mr. Pitcher's method differed in few important particulars 

 from that employed by the writer in his " Spectro-photometric 

 Study of Pigments," already described in these pages.* The 

 spectro-photometer was one of Glan's pattern, in which the 

 very ingenious, but far from sensitive, polarizing apparatus 

 had been removed ; its place being supplied by a pair of NicoPs 

 prisms before the slit. To the right hand of the spectroscope 

 slit and distant from the same about 60 cm , was placed the com- 

 parison flame, an Argand gas burner. Its rays, having been 

 rendered parallel by passage through a lens of short focal 

 length, passed through the two Mcol's prisms already referred 



* This Journal, vol. xxviii, p. 342. 



