NICHOLS. 



THE VISIBLE RADIATION FROM CARBON. 



101 



melting-point was again tested in the manner just described. The result 

 of this determination was 1687°. The latter reading was, I think, too 

 high, since subsequent examination under the microscope showed that 

 the loop of the wire behind the junction had been melted so that the 

 junction was probably a few degrees too hot. It may safely be conclude 1 

 from these determinations that the melting-point of the Wollaston wire 

 was at least one hundred degrees lower than that of pure platinum. 



Method of Checking the Constancy of the Acetylene Flame. 



To secure as complete a check as possible upon the constancy of the 

 flame, the following method, based upon the assumption that so long as 

 the radiation from the flame remained constant, its light-giving power 



dd 



Ifll 



Figure 16. 



would not vary, w r as employed. A diaphragm (d, Figure 16) similar to 

 that interposed between the slit and the flame, and having an aperture of 

 the same size, and mounted on the opposite side of the latter and a thermo- 

 pile p, was placed at a distance of about 15 cm. from this opening. A 

 second diaphragm, d' , with an intervening air space, served to cut off, in 

 large part, the radiation from the heated metal. Two thin sheets of 

 glass forming the sides of an empty cell c, of the kind used in the study 

 of absorption spectra, etc., were placed between the cone of the thermo- 

 pile and the second diaphragm ; so that only those rays from the (lame 

 which were transmitted by the glass fell upon the face of the pile. 



The thermopile was connected with a sensitive d'Arsonval galvano- 

 meter g, the circuit being kept permanently closed ; and a double metallic 

 shutter s, which could be raised or lowered so as to open or close the 

 opening in the diaphragm next to the flame, was so mounted that it could 

 be readily operated by an observer at the telescope of the galvanometer. 

 When a reading of the radiation from the flame was to be made, the 

 zero point of the galvanometer was noted, and this shutter was raised 

 during the short interval of time necessary to bring the needle, which 

 was Dot strongly damped, to its first turning point. The shutter was 



