METHODS OF STUDYING VISION IN ANIMALS 13 



psychology. Nevertheless, it seems highly desirable that energy 

 measurements be made to supplement our photometric data. 



Of the various radiometric devices and methods ' which 

 are at our command, the radiomicrometer and the selenium 

 cell have been at least partially adapted to our needs and tested 

 in investigations which are now in progress. 



Concerning the former instrument Coblentz writes: "The 

 radiomicrometer is essentially a moving coil galvanometer 

 having a single loop of wire with a thermo-junction at one end. 

 This instrument was invented independently by d'Arsonval ^ 

 and by Boys.^ The former used a loop, one part of which 

 was silver and the other was of palladium. The latter used a 

 junction of bismuth and antimony, which was soldered to a 

 loop of copper wire. 



" The sensibility of the Boys instrument was given as lo ouV ooo 

 to 93-ooVooo of 1°. From subsequent work with other radia- 

 tion meters in which this high degree of sensitiveness has never 

 been attained, it would appear that the sensibility of the radio- 

 micrometer was overestimated." * 



Professor G. H. Parker is at present using for the measuring 

 of photic stimuli, as applied to organisms without highly de- 

 veloped visual organs, the Boys radiomicrometer essentially 

 as it is described by Adams. ^ As he later will publish a de- 

 scription of the method, with the results of his experiments, 

 we need add merely a comment. 



The radiomicrometer is certain to prove useful in connection 

 with studies of light vision. It is not sufficiently sensitive to 

 enable us to measure directly extremely weak stimuli (threshold 

 or approximations thereto), and it can not be used at all unless 

 a highly stable base is available. 



The selenium cell '' has been classed among the " selective 



' Coblentz, W. W. Instruments and methods used in radiometry. Bull. U. S. 

 Bureau of Standards, 1908, vol. 4, pp. 391-460. 



2 d'Arsonval. Soe. Franc, de Phys., 1886, pp. 30, 77. 



^ Boys, C. V. Phil. Trans., vol. 180, pp. 159. 



* Coblentz. Loc. cit., p. 395. 



^ Adams, J. M. The transmission of roentgen rays through metallic sheets. 

 Proc. Amer. Acad., 1907, vol. 42, p. 673 ff. 



" Pfund, A. H. The electrical and optical properties of metallic selenium. Phys- 

 ical Review, 1909, vol. 28, pp. 324-336. 



Stebbins, Joel. The color-sensibility of selenium in cells. Astrophysical Jour- 

 nal, 1908, vol. 27, pp. 183-187. 



