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IOWA ACADEMY OF SCIENCES 
Angstrom unit could be placed separately upon the slit of the echelon 
spectrometer. To facilitate the arrangement and manipulation of the 
apparatus a plane mirror was introduced between the colimator and the 
prisms, which permitted of rotation so that any line of the prism spec- 
trum could be placed upon the slit of the echelon spectrometer. The 
echelon consisted of 30 plates, 5 mm, thick and 1 mm. width of step. 
This would make the order of spectrum employed of the order of 5000, 
which v/ould make the dispersion several times that of the grating spec- 
trometer previously employed. The eye piece of the echelon spectrometer 
was fitted with a plate holder, for photographic work. 
Considerable work has been done by eye observation and -some 200 
negatives were taken of the most interesting lines of the iron spectrum 
but as bearing upon the pressure shift they only contribute evidence 
confirming the unsymmetrical broadening toward the red. 
1 Ap. Jour. 6 ; 169 ; 1897. 
2 Ap. Jour. 15; 132; 1902. 
3 Ap. Jour. 17 ; 154 ; 1903. 
