1 76 STUDIES IN luminescence;. 



glass was found to be about 40 per cent. Its transmission throughout the 

 range of wave-lengths used in our measurements was not measurably 

 selective. 



To determine the distribution of energy in the spectrum of the light 

 received from the comparison flame after passing through the ground glass 

 and the optical parts of the spectrophotometer, this spectrum was carefully 

 compared wave-length by wave-length with the light received through the 

 other collimator of the instrument from a black body of known temperature. 



The black body, Fig. 171, consisted of a tube of Acheson graphite about 

 50 cm. long, of 1.7 cm. bore and 4.0 cm. external diameter. In the middle 

 this tube was turned down for about 20 cm. until the thickness of the walls 

 was reduced to about 0.4 cm. and the thin-walled cylindrical chamber thus 

 formed was heated by means of an alternating electric current furnished by 

 a step-down transformer, of whose secondary circuit it formed the principal 

 part. 



The ends of the cylindrical body were graphite plugs, each with an axial 

 hole 1 cm. in diameter. Through one of these passed a tube of fused quartz 

 containing a platinurn-rhodium-platinum thermo-junction of wires which 

 had been calibrated at the Bureau of Standards. This junction received 

 radiation from the surrounding walls and could be pushed in and out at 

 will so as to ascertain the range of temperatures within the black body. 

 Through the opening in the 

 other plug and through cor- 

 responding openings in dia- 

 phragms located nearer the pj g I7I 

 ends of the graphite tube, 



light from the incandescent tip of the quartz tube reached the spectro- 

 photometer. 



To reduce heat losses and prevent the too rapid oxidation of the graphite, 

 the tube was embedded to a depth of about 8 cm. in a mass of powered 

 magnesite, which was contained in a hollow cylinder of magnesium oxide 

 and asbestos such as is used for the packing of large steam pipes. 



When the primary circuit was supplied with 80 amperes at no volts the 

 temperature of this improvised furnace, as indicated by the thermo-junction, 

 rose slowly to nearly 1500 C, at which temperature it remained with little 

 change for a considerable time. Temperatures were determined in the 

 usual way by means of a potentiometer and cadmium cell. 



The arrangement of the apparatus is shown in Fig. 172, in which R is an 

 adjustable resistance, A is an a. c. ammeter, P is the primary coil of trans- 

 former, S is the secondary coil, D is a Dewar flask with the cold junction in 

 ice, a and b are the collimator slits of the spectrophotometer, L is the 

 Lummer-Brodhun prism of the spectrophotometer, is the observing 

 telescope of the spectrophotometer, g is the ground glass in front of slit b. 



The slits a and b were set once for all to convenient widths, b being 30 

 divisions =0.6 mm. in width, and a 0.06 mm. The adjustable diaphragm 

 in was of the same width as slit b. 



In this determination one observer made settings for wave-length and 

 watched the contrast fields of the spectrophotometer, while another recorded 

 the positions of the comparison flame when, for each region of the spectrum, 



