84 INFRA-RED EMISSION SPECTRA. 



spectrometer, containing the collimating mirrors, and the Wadsworth 

 mirror-prism outfit, were entirely inclosed by a thin sheet-metal box, lined 

 with black velvet. The spectrometer slit was covered with a clear plate 

 of fluorite. The openings in the top, for adjusting the mirrors, were closed 

 with soft wax, while the hole admitting the axis for rotating the mirror- 

 prism table was tightened with a nut and packing. 



Within the box, and below the level of the mirrors, were placed vessels 

 containing phosphorous pentoxide and sticks of potassium hydroxide, 

 which entirely eliminated the absorption bands of C0 2 , and water-vapor 

 from the emission curves. A water-cooled shutter was placed before the 

 spectrometer slit, and the Nernst glower, inclosed in an asbestos case to 

 prevent air-currents, was placed close to the shutter. Observations were 

 also made without inclosing the glower, to prove that the effects observed 

 are not due to stray radiation from the asbestos case. In spite of all that 

 has been written about bolometers, it may be added that there was no 

 "drift," unless a poor storage battery was accidentally used. 



The auxiliary galvanometer of 5.3 ohms resistance, with a single swing 

 of 4 to 5 seconds, had a current sensibility of i=i.6 to 1.5 Xio~ 10 ampere. 

 A greater sensitiveness for the same period would have been possible, by 

 using a lighter suspension. The present suspension of ten needles was 

 just heavy enough so as not to be affected by tremors. 



Using a battery current of 0.04 ampere the computed temperature 

 sensibility was 5Xio~ 5 C, which was generally far in excess of that re- 

 quired. The deflections were reduced to 14 to 15 cm., by inserting resist- 

 ance in series with the galvanometer. The individual readings would 

 vary by 1 mm. or less than 1 per cent, which is as close as the nature of 

 the work required, since the actual deflections were as high as 2000 cm. 

 Furthermore, at high temperatures (especially when operated above nor- 

 mal power consumption) the filament would deteriorate in emission by 

 that amount during the series of observations. 



The calibration curve of the fluorite prism was constructed from the 

 refractive indices, found by Paschen, 1 which, after plotting all the obser- 

 vations made by different observers, seems to be as close to the most 

 probable values as observations will permit. Unfortunately the dispersion 

 curve passes through a double curvature at 1.5 //, just where the energy 

 spectra have their maximum. In the region of 1.5 ,, the correction for 

 purity, so-called "slit-width" correction, is a maximum. 



The wave-lengths in the calibration curve were plotted to the fourth 

 decimal place, so that there is a certainty of the values to at least the 

 second decimal place. This, however, is of less importance than the 

 value of the slit-width correction, which was made according to Paschen, 2 

 the values being obtained from a curve plotted on a large scale to insure 



1 Paschen: Ann. der Phys. (4), 4, p. 299, 1901. 



2 Paschen: Ann. der Phys. (3), 60, p. 714, 1897. 



