[plaskett] 



THE SPECTROSCOPIC BINARY fl ORIONIS 



217 



trum, was to obtain several plates on each night the star was observed 

 and use their mean velocity as the velocity of the star at their mean 

 epoch. As the star is bright, a spectrum can be obtained in ten minutes 

 or less with the three prism spectrograph and in about two minutes with 

 one prism. Consequently not much time is required to obtain half a 

 dozen plates and, unless the period is very short, no error due to change 

 of phase can enter. The probable error of a night's observation will by 

 this moans be considerably reduced and a much better chance obtained 

 of determining its period of variation. 



Plates were accordingly obtained whenever possible until the star 

 became inaccessible in April, 1908, and observations were continued 

 during the present season until March 23, 1909. Owing to the very 

 smoky and cloudy weather last fall very few plates were obtained until 

 December. In all 273 plates obtained on 54 nights have been used in 

 this discussion. Of these 150 were made with a dispersion of three 

 prisms, 123 with one prism. The investigation on slit width ^ showed 

 that lower probable errors were obtained with the higher dispersion and 

 it was used wherever possible. However, the star was also observed with 

 the one prism spectrograph when our programme would not permit the 

 use of three prisms. Three-fourths of the observations and all the mea- 

 surements were made by myself in order to avoid as far as possible any 

 chance of systematic discrepancies. 



The star ^ Orionis is of the helium type, Miss Maury's VI c, and 

 has fairly well defined lines of hydrogen and helium,^ the magnesium 

 À 4481. and the calcium H ami K. It also contains lines due to silicon, 

 oxygen and carbon and a few faint metallic lines. In my early measures 

 for slit width effect 8 lines were measured in the three prism plates and 

 7 in the single prism plates. 



Lines Measured in /? Orionis. 



1 Report of Chief Astronomer, 1907-1908, p. 86 ; also Astrophysical Journal, 

 Vol. XXVIII, p. 259, November, 1908. 



