SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 68 



(') measured from the sharp maximum in the spectrum of the lamp 

 at 1.838 II. The true deviations, B, equal 39° 33.7' + A^ for the 60° 

 rock-salt prism and 8° 9.9' + A^ for the small rock-salt prism. The 

 plate distances, d, are in cm. measured from the position of the 

 maximum at 1.838 /* as zero. d^/dA, given on an arbitrary scale, is 

 for use in transforming the energy curves from the prismatic to the 

 normal scale, dd/da, is the change in deviation, d, in cm. for a 

 change in the angle of the prism of one degree. The deviations were 

 computed from the indices of refraction given by Paschen.' 



Table 2. — Dispersion Data 

 60° rock-salt prism 



X in /x. . , 

 Ae in ' . . 

 d in cm. 

 de/d\.., 



15° 18.4 ROCK-SALT PRISM 



X in M 1 2 



\e in ' ... .1 — 5.2 +0.4 

 d in cm... .-1.3' o.i 



d^/dX (334) (i6oo) 



dd/da 0.09 o.oi 



10 

 170.0 



33.9 

 491 



10 



30.2 



7.6 

 648 

 0.53 



X in At- ■ . 

 A6» in '.. 

 d in cm.. 

 d(9/dX.. . 

 dd/da... 



20 



134-4 



33.6 



262 



2.35 



ADJUSTMENTS OF APPARATUS 

 Each day's adjustments were as follows : A single Nernst glower 

 was lit and placed in the center of the position to be occupied by the 

 larger lamp in front of the slit. Mirror M^ (see fig. 2) was then so 

 adjusted that the range over which the filament was visible in it, 

 as looked at from just in front of Mo, was centrally situated with 

 regard to Mo. Next Mo was adjusted so that the return beam cen- 

 trally filled M^. This generally caused the image of the lamp to 



^ Annalen der Physik, 26, p. 120, 1908. The following were computed from 

 the formula I on p. 130 1. c. : 



X 1.838A 

 Log n 0.1839026 



14A I ISA i6/tt j I7A I i8/it I 19M 



0.1649292 0.1618148 0.1583802 0.1545992 0. 1504420 ' 0.145874s 



2011 21 ^'. 



0.1408578 0.1341792 



