NO. 17 ULTRA-VIOLET INTENSITIES McALISTER 3 



the monochromator observations. A Leeds and Northrup H.S. 

 galvanometer with a curved (circular) scale was used and calibrated 

 for current versus deflection in the usual manner. The thermocouple 

 galvanometer combination was calibrated in terms of a radiation 

 standard furnished by the Bureau of Standards. By using an auxiliary 

 windowless thermocouple and a monochromatic beam of light — 

 A 4,358 through the monochromator of which the window of the 

 vacuum couple transmitted 91.0 per cent — it was determined that 

 86 per cent of the radiant energy from the standard of radiation was 

 transmitted by the window of the vacuum thermocouple. The thermo- 

 couple was provided with a slit o.io mm wide, which is immediately 

 in front of the receiver. The receiver is 2.8 mm high. At a distance 

 of 2 m from the standard of radiation the flux is 64.2 microwatts per 

 cm'. This produced a deflection of 41.5 mm. Hence a deflection of 

 i.o mm corresponds to an intensity of 1.33 microwatts per cm^ on 

 the receiver. For the bare thermocouple a deflection of i.o mm corre- 

 sponds to an intensity of 8.51 microwatts per cm ^ 



For dispersion two Bausch & Lomb quartz monochromators were 

 used, both singly and in tandem as a double monochromator. When 

 used singly the exit slit of the monochromator was removed and 

 replaced by the thermocouple which is provided with a slit. When 

 the two were used in tandem the curved exit slit of the first mono- 

 chromator served as the entrant slit of the second. This arrangement 

 straightens out the image falling on the thermocouple and improves 

 the resolution. The two w^ere maintained in permanent alignment 

 on a cast-iron table with seats provided for the leveling screws of 

 the monochromators. A special sleeve held the collimator of the 

 second instrument in proper position with respect to the telescope 

 of the first. The use of two instruments greatly reduces the scattered 

 light which in an instrument of numerical aperture f4 is to a certain 

 extent inevitable. The source to be examined was rigidly attached 

 to the cast-iron table so that no change in illumination could occur 

 during a series of observations. The length of the entrant slit was 

 reduced to 2.75 mm by two knife edges and the arc placed 250 mm 

 from it. No condensing lens was used. Since the section of the arc 

 exposed was nearly square the illumination produced at the collimator 

 lens of the monochromator was square in form and covered about 

 one half the area of the aperture. The corners of the square were 

 well inside the circle limiting the aperture, the illumination being 

 observed by placing the eye at the exit slit with an intervening screen 

 to reduce the intensity. In measuring the absolute intensities of 

 isolated spectral lines a single monochromator with a o.io-mm slit 



