356 10 



which can be seen on the image of Alcyone on Figure 2, are strongly eclipsed by 

 the brightness of the main image and show a somewhat irregular figure owing to 

 their inclined position. Still it may be noted as a support of the assumption made 

 above, that also these additional spectra give practically the right effective wave- 

 length, while they are about 9'7 magnitudes fainter than the common spectra of 

 the P' order. For the 5 brightest Pleiades these by-spectra gave on 42 images in 

 the mean Z = r054 mm ±'002 mm (m. e.) or I, = —"30 ±'04 (m. e.) in comparison 

 with — '20 as found from the rest of the material. The difference '10 ±04 (m. e.) 

 is not too large. If anything, it would indicate a shortening of the effective wave- 

 length for decreasing intensity of the light and correspondingly prolonged exposure 

 time and that is exactly the opposite of what was feared. 



The reduction of the measures was made in the following way. The distance 

 in millimeters between the two spectra to the right and to the left of the central 



Table 2. 



Corrections to normal strength of the image, spectra of P' order, observed values, 

 diameter of central Image, d -05 -06 .07 -08 -09 -10 -11 -12 -13 -14 mm 



mean correction ot the effective wave-) ,„-,„- 



; 12-U lO-o 0-7 4-4 1-9 — 1-6 — S'S — o'l — o-9u 



length to a = -10 mm ) 



number of differences used 125 171 330 418 164 — 120 400 220 131 



diameter of central image, d -15 -16 -17 -18 -19 -20 -21 -22 -23 •24 mm 



mean correction of the effective wave- |„ ,„ „„ „ „„ 



—6-7 — 4-8 —7-6 —6-7 —7-3 — S-8 — 8-2 —7-1 — 4-8 —7-1 u 

 length to a = -10 mm ) 



number of differences used 95 132 138 105 74 41 25 13 6 7 



image, or the effective wavelength was first corrected to the zenith for selective 

 extinction of the light in the earth's atmosphere. These corrections are given in 

 Table 1 for each separate plate. 



The intensity of the image on the plate was indicated by the diameter d of 

 the central image of the star. As normal intensity was considered that for which 

 the diameter of the central star image was -^/lo mm. In that case the spectra are 

 about 2 magnitudes stronger than the first visible traces and are therefore just well 

 exposed. 



It remained to find for a given star the variation of the effective wavelength 

 with changing intensity of the spectra as shown on the plate. To this end for each 

 star the differences of the observed effective wavelengths from that corresponding 

 to a diameter cf = "1 mm of the central image were formed. The mean values of 

 these de%iations for different Aalues of d are given in Table 2. No appreciable 

 differences were found in these corrections for stars of different colour. The finally 

 adopted smoothed corrections are given in Table 3. They have been applied to 

 each separate effective wavelength. 



Owing to a mistake the effective wavelengths derived from the spectra of the 

 P' order were treated separate!}' for those exposures where also spectra of higher 



