Sl\l.l<;il T AND ITS MEASUREMENT 207 



Practically all glasses, regardless of their color are very trans- 

 parent to infra-red radiation. That the transmission curve 

 sinks in the red portion of the visible spectrum is no guarantee 

 that it will not rise again in the infra-red. Much the same 

 statement applies to the ultra-violet; ordinary glass, even in 

 moderately thick layers, transmits as far as wave length 0.35 y. 

 or 0.34 ix, and this may be true for a glass giving very complete 

 absorption in the blue. 59 Moreover a glass does not show high 

 transmission in a certain portion of the spectrum with almost 

 complete absorption in immediately adjacent portions. In- 

 stead, it exhibits a transmission curve that often is of much the 

 same general appearance as the curves for darkening of silver 

 chloride-gelatine preparations shown in figure 5, or the sensi- 

 tivity curve in the same figure (although a transmission curve 

 may show more than one maximum, even in the visible portion 

 of the spectrum). 



The ordinates of such a transmission curve alter with changing 

 thickness of glass (as is true of any absorbing medium — Lambert's 

 law, Beer's law), the function being exponential and not linear 

 so that a given piece of glass does not absorb twice as much as a 

 piece of the same glass half as thick. This law only holds for 

 monochromatic radiation. Lambert's law to the effect that 

 the . percentage of radiation absorbed by a given medium of 

 given thickness is the same for all intensities of the incident 

 radiation is also true only for single wave lengths or small 

 spectral regions. When the radiation is complex, as with sun- 

 light, the result is the absorption curve mentioned above. This 

 must be determined for each piece of glass because it is at pres- 

 ent impossible to produce sheets of glass that are alike in their 

 absorbing power. The losses due to reflection from the sur- 

 faces of the glass plate (it must be remembered that a single 

 plate of glass has two surfaces and that the entering ray also 

 suffers multiple reflection between them) are usually calculated 

 from Fresnel's law. 1 Even if the simplest form of actinometer 



69 Ham, W. R., R. B. Fehr, and R. E. Bitner, The absorption of ultra-violet 

 by various kinds of glass. Penn. State College Report 1914. Part I: 158-162. 

 Published 1916. 





