206 HOWARD E. PULLING 



and although the instruments cannot be said to be well enough 

 understood for general biological use, their value for some 

 forms of laboratory work is undoubted. 



Photo-voltaic cells and photo-electrolytic cells are of unknown 

 value. 



In conclusion a word may be said regarding means for trans- 

 mitting only portions of the spectrum to radiation-sensitive 

 instruments: dispersion and differential transmission. 



Dispersion is accomplished by refracting the radiation by a 

 prism or by reflecting it from, or passing it through, a grating. 6 

 As has been pointed out, in a prismatic spectrum the original 

 intensity has been unequally altered by passage through a 

 prism. This deviation effect may be corrected by using a train 

 of prisms, the secondaries correcting the deviation while still 

 allowing dispersion, so that the " direct vision" spectroscope 

 yields a normal spectrum. There are many objections to these 

 spectroscopes and the simplest and, in general, most reliable 

 means of dispersion is the single prism. The data obtained 

 with such a prism are easily corrected for deviation to yield 

 results such as would have been obtained if a normal spectrum 

 had actually been obtained in the first place. Gratings reduce 

 the intensity of radiation more than do prisms because they 

 yield many spectra and because of losses by extinction; they 

 are expensive to rule and have had little use except in very 

 precise work of certain kinds. 



Isolation of portions of a spectrum by absorption of the re- 

 maining radiation may be accomplished by interposing suitable 

 screens, either solid or liquid, between the instrument and the 

 light source. 38 The solid screens may be thin layers of some ab- 

 sorbent element or compound, as a thin layer of gold for pre- 

 venting transmission of infra-red radiation. Colored glasses are 

 frequently used for the same purpose. It must be remembered, 

 however, that the color of a screen is no insurance that it trans- 

 mits only radiation between the wave length limits of that color. 



5S See, for example Mees, C. E. K., Light filters for photometry. Trans. 

 Ilium. Eng. Soc. 9: 990-998. 1914. 



Ives, II. Jv, Artificial daylight. J. Franklin Inst. 177: 471-499. 1914. 



