NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 87 



excess of oxygen, and even where the metal to be melted is platinum 

 itself, fusion occurs before the luminous portion of the flame, the action 

 of which upon the thermo-electric properties of the junction is to be 

 feared, has been reached. It is well-known that a junction, the perform- 

 ance of which has been vitiated by exposure to the vapors of a thane or 

 furnace, can be restored to its original condition by immersion in an 

 oxydizing flame. In this method of calibration the junction is continually 

 subject to such oxidation as is necessary to preserve it. Thus one of the 

 sources of error which it has been found most difficult to guard against 

 in the use of the furnace is altogether avoided. 



Figure 8 contains the calibration curve of the thermo-elements used in 

 this investigation, and likewise, for purpose of comparison, a curve repro- 

 duced from Waggener's paper and extrapolated by him from data given 

 by Holborn and Wien. It will be seen that while the curves are not 

 identical they are of the same character, and that the differences are not 

 greater than experience would lead us to expect in the case of different 

 thermo-elements, even where these are from metals of the same manufac- 

 ture. It is not a question of absolute electro-motive forces, but of the 

 form of the curves, since what we need is a criterion by means of which 

 to determine whether temperature readings based upon Violle's values for 

 palladium and platinum are in reasonable accord with those obtained by 

 the extension of the curve of Holborn and Wien. 



The Spectrophotometer . 



The spectrophotometer used was a copy of the instrument designed 

 by Lummer and Brodhun for the Imperial Institute in Charlotteuburg. 

 It consists of a one-prism spectroscope with two collimator tubes, placed 

 at right-angles to each other, as shown in Figure 9. Each of these tubes 

 carries a slit the width of which is regulated by means of an accurate 

 micrometer screw with a drum head divided into one hundred parts. By 

 estimating tenths of a scale division, the width of the slits could be esti- 

 mated to one one-thousandth of a revolution. 



The essential feature of this photometer consists in the Lummer-Brod- 

 hun prism D, placed between the objective lenses of the two collimators, 

 and the dispersing prism in such a position that the beam of light from 

 one of the tubes is transmitted directly to the latter, while that from the 

 other tube is bent to 90° by total reflection. The instrument was set up 

 with collimator A in such a position that a portion of the surface of the 

 incandescent rod lying nearest to the point at which the thermo-eleinent 

 had been inserted formed a field of illumination for the slit at a distance 



