AS STUDIED BY MEANS OF THE RADIOMICROMETER. 31 



When the thermo-j unction was exposed to the radiation and the source of 

 energy removed, the instrument returned to its original zero position to 

 within 0.5 mm. In most cases several readings were made for a given 

 amount of radiation, and these usually agreed to within 1 per cent. The 

 source of energy was a Nernst glower attached to a storage battery, the 

 amperage being 1.2 and the voltage 110. This was found to be very con- 

 stant, successive readings in the same position of the spectra agreeing well 

 with one another. 



The vessels used for holding the solutions were made of brass and gold 

 plated. They were about 4 cm. in diameter and of the desired thickness. 

 The ends were made of the best optical glass. Vessels of the same thickness 

 gave practically the same deflection both when empty and when filled with 

 water. 



METHOD OF PROCEDURE. 



The light from a Nernst glower was rendered parallel by a lens, then 

 passed through the vessel containing the solution, and allowed to fall on the 

 slit of a Hilger spectroscope. The solution was first inserted into the path of 

 the light, and then the pure solvent, this being done without disturbing the 

 adjustment. By means of a movable framework, first the vessel containing 

 the solution and then that containing the solvent were interposed in the path 

 of the beam. A metal screen interposed between the Nernst glower and the 

 vessel containing the solution allowed the light to pass through the solution 

 only when an observation was being made. By this means the thermo- 

 electric junction was exposed to the radiation only long enough to read the 

 deflection of the mirror. 



The light, after passing through the solution and the slit of the spectro- 

 scope, fell upon the prism of the Hilger spectroscope. A second slit was 

 inserted in the spectroscope instead of the eye-piece. The light passed from 

 the prism through this second slit, and was then focused on the thermal 

 junction of the radiomicrometer. 



The Hilger spectroscope contained a milled head, graduated so that the 

 wave-lengths could be read off directly. By suitably turning this head 

 any desired wave-length could be thrown upon the junction of the radio- 

 micrometer. 



The width of the slit used in the visible part of the spectrum was 0.4 mm. 

 In the infra-red, where there is far more energy, the slit width was cut down 

 to 0.22 mm. A series of readings was carried out as follows: The vessel 

 containing the solvent was first placed in the path of the beam of light, the 

 screen removed, and the deflection of the mirror noted. Then the vessel 

 containing the solution was put in the same place that was formerly occupied 

 by the vessel containing the solvent, the screen removed, and the deflection 

 again noted. The prism was then turned slightly by means of the gradu- 

 ated and calibrated head, and a new wave-length allowed to fall on the junc- 



