SUNLIGHT AND ITS MEASUREMENT 209 



changes in concentration and in thickness of the absorbing 

 layer) and hence the transmission curve may be changed" at 

 will and be duplicated. Some solutions exhibit very abrupt 

 maxima in small spectral regions — transmission bands — so that 

 for most purposes the radiation passing through them may 

 be regarded as monochromatic. Aniline dyes and their com- 

 binations offer excellent banded transmissions in almost all 

 portions of the spectrum, but they alter in solution upon ex- 

 posure to radiation and fade quite rapidly if the insolation is 

 intense. Solutions of inorganic substances are usually stable 

 and offer a considerable range of choice of rather sharply banded 

 spectra. 62 



It may be noted that if the surface is always exposed at right 

 angles to the incident beam and the transmission is always 

 limited to the same small portion of the spectrum by the same 

 prism or screen properly placed with respect to the beam, the 

 percentage of radiation actually effective may be determined 

 for the entire system once for all by experimental means, with- 

 out having recourse to involved calculations which become in- 

 creasingly difficult to evaluate the greater the number of surfaces 

 involved. 



- Watts, W. M., Index of spectra. Appendix a-w. 2nd ed. Manchester, 

 England, 1889. 



Eckert, F., and R. Pummerer, Photographische Spektralphotometrie der 

 Absorptionspektren von Farbstoffcn. Zs. Phys. Chem. 87: 599-618. 1914. 



Jones, H. C, and J. A. Anderson, The absorption spectra of solutions. Car- 

 negie Inst. Wash. Pub. 110. 19C9. 



Jones, H. C, and W. W. Strong, A study of the absorption spectra of solutions 

 of certain salts of potassium, cobalt, nickel, copper, chromium, erbium, prase- 

 odymium, neodymium and uranium as affected by chemical agents and by 

 temperature. Carnegie Inst. Wash. Pub. 130. 1910. 



Jones, H. C, and J. S. Guy, The absorption spectra of solutions as affected 

 by temperature and by dilution: a quantitative study of absorption spectra by 

 means of the radio-micrometer. Carnegie Inst. Wash. Pub. 190. 1913. 



Wood's Optics cited in note 1. 



