208 HOWARD E. PULLING 



is used (a piece of silver-chloride-gelatine paper exposed directly 

 without a screen of any sort) reflection takes place at the sur- 

 face and unless the radiation is always incident at right angles 

 to the plane of the paper the effect of these reflections is dif- 

 ficult to compute and, since the paper is constantly changing 

 in tint, it varies in its reflections of the radiation in the various 

 wave lengths composing sunlight, the matter becoming still more 

 complex with polarized sky radiation. 



Recently colored glasses have been manufactured that show 

 rather sharply delimited transmission bands in which the trans- 

 mission curve rises very abruptly to a flat-topped maximum. 

 As such glasses are available by which, either singly or in com- 

 bination, narrow transmission bands can be obtained in almost 

 any portion of the visible spectrum, 60 they are convenient for 

 many sorts of spectro-radiometric work. Upon exposure to 

 intense radiation the transmission may, however, drop 15 or 

 20% at some wave lengths (the effect is not uniform at all 

 portions of the spectrum for any piece of colored glass) due 

 to the absorption of heat. 01 Rises in temperature of less than 

 40°C, have very small effects for most of these glasses, however, 

 and by interposing a piece of clear glass between the sun and 

 the colored glass the rise in temperature will not be excessive. 



Liquid solutions are much like glasses in their general prop- 

 erties but have the advantage that the user can himself more 

 easily vary the amount of absorption by small increments or 

 decrements than is the case for glasses. The degree of absorp- 

 tion depends upon the total amount of dissolved absorbing 

 substance encountered by the beam (thus being altered by both 



60 Gage, H. P., Colored glass in illuminating engineering. Trans. Ilium. 

 Eng. Soc. 11: 1050-1067. 1916. 



Coblentz, W. W., W. B. Emerson and M. B. Long, Spectra-radiometrio 

 investigation of the transmission of various substances. U. S. Bur. Standards 

 Sci. paper 325. 1919. 



01 Gibson, K. S., The effect of temperature upon the coefficient of absorption 

 of certain colored glasses of known composition. Phys. Rev. (2) 7: 191-202. 

 1916. 



Luckeish, M., The influence of temperature on the transmission factor of 

 colored glasses. J. Franklin Inst. 187: 225-226. 1919. 



