350 



for the determination of effective wavelengths. I have therefore diminished the 

 number of plates necessary to cover the whole group by diaphragming the reflector 

 down to an aperture of 40 inclies or 1 meter. In this way the diameter of the effi- 

 cient field is doubled to about 40', while on the other hand for stars near the 

 axis 1 magnitude is lost. Still it could be foreseen, that in half an hour of exposure 

 time stars of the 14"^ magnitude would be reached. — The 68 plates of the pro- 

 gramme have all been taken. 



The place of the reflector is 1^ 52"^ W. of Grw., 34" 13' north latitude and 

 1730 meter above sea level. Hence the thickness of the atmosphere is '81 of that 



at sea level. The focal length of the re- 

 flector is 7606 mm, hence 1mm on the 

 plate is equal to 27"'12. 



The grating used consisted of black 

 overspun, stretched rubber wires of a dia- 

 meter d = 2'745 mm separated by intervals 

 I = 3'255 mm. The spectra of uneven order 

 are thus practically (if d = I exactl}') at 

 their maximum intensity. The constant of 

 the grating, d+l, was exactly 6 mm. 



2. Plate Material. 



The longest exposure used was 30 min. 

 or 1800 sec. In order to make it possible 

 to measure all stars at about the same 

 p;„m,g J intensity of the image further exposures of 



570, 180, 57, 18, 6 and 2 sec. were made. 

 The proportion between two consecutive exposures, ((/ÏO is nearly equal to 19:6), 

 corresponds to a difference in brightness of about 1 magnitude. The four shortest 

 exposures were made successively on the same plate. Each series of exposures 

 therefore consists of 4 plates. The brightest star, Alcyone, was taken as centre of 

 the group. The positions of the centres of the separate fields are shown in Figure 1. 

 The limits of this figure are the same as of the map given at the end of this paper. 

 On each of the 2 X 7 = 14 inner fields a series of 4 plates, as indicated above, was 

 exposed. Of the 12 + 1 = 13 outer fields only one plate of 30 min. exposure time 

 was taken. The programme comprised 4x 14-rl2 = 68 plates. To this was added 

 one duplicate plate. Hence a total number of 69 plates was used. A catalogue of 

 these 69 plates is given in Table 1. 



As the diffraction images of higher order, especially of the brighter stars, are 

 rather extended, neighbouring images occasionally overlap in one position of the 

 grating and not in another. Accordingly the grating was used in two different 

 orientations. On 1912 Aug. 10, Sept. 8, 9 and 11 and Oct. 16 the wires of the grating 



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 X 



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