504 Prof. Angstrom on the Intensity of 



platinum and smoke. The four branches of the Wheatstone- 

 bridge arrangement, of which the gratings formed two, had 

 each a resistance of about 5 ohms. In order to be able to 

 rapidly test the sensitiveness of the combination, a constant 

 resistance was introduced as a secondary circuit into one of 

 the branches. The opening or closing of this circuit usually 

 made a difference of 75 scale-divisions. If not, the reading 

 was reduced to that standard sensitiveness. 



The bolometer was separated from the end of the discharge- 

 tube by a double screen with a perforation, inside which was 

 suspended a small screen. This was quickly pulled up to 

 expose the bolometer. The strength of current through the 

 discharge-tube was measured by the galvanometer, the diffe- 

 rence of potential within it by the electrometer, and the 

 deflexion of the galvanometer in the bolometer circuit was 

 read from minute to minute. The latter gradually increased, 

 owing to the warming of the walls of the discharge-tube. By 

 suddenly breaking the current and again observing the 

 bolometer the radiation of the tube- walls was eliminated. 

 Another method of elimination was by interposing a plate of 

 alum about 4 millim. thick, which totally absorbed the radia- 

 tion from the glass. Another source of error was the reflexion 

 from the walls of the tube. The end of the tube opposite the 

 bolometer was closed by a plane-parallel plate of rock-salt. 

 This occasioned a loss by reflexion, whereas the other surfaces 

 entailed a gain. Both were corrected by introducing a small 

 copper box heated by steam circulation into a tube of the 

 same construction as the discharge-tube, observing the 

 bolometer deflexions, and repeating with the box alone. 



Prof. Angstrom states his main results as follows : — 



1. For a given gas and a given pressure the radiation of 

 the positive light is proportional to the intensity of the electric 

 current. 



2. For a given gas and pressure the composition of the 

 radiation is constant and independent of the strength of 

 current. 



3. On increasing the pressure of the gas, the total radiation 

 for a given strength of current increases as a rule, slowly at 

 low, more rapidly at high pressures. At the same time the 

 composition of the radiation changes, inasmuch as the ratio 

 of the intensity of the shorter waves to the total radiation 

 decreases. Thus the distribution of the intensity in the 

 spectrum changes in such a manner that with diminishing 

 pressure the intensity of radiation increases for the shorter 

 wave-lengths. 



4. The ratio between the intensity of total radiation and 



