352 



Table 1 (continued). 



no. 



date 



centre of field 



sidereal time 



efficiency 





o 



a 

 o 



the 

 lilh 



oV.a-3 



of 



a 



â 



of exposure 



surt 

 ctiv 

 eng 







(1900) 



(1900) 





of plate 



1 p 







c B "o 



plates 





= i!= > 







3h 





middle duration 





■■^ 



o' 



»--1 



£ = " ? 





1912 



m 



o / 



h m 



s 



m 



m 





m 





221 



Oct. 12 



43-2 



23 35 



3 56 



57, 18, 6, 2 



5-5,4-8,3-9,2-9 



—3 









137 



222 





43-2 



23 35 



4 14 



1800 



90 



+ •2 









232 



223 



— — 



43-2 



24 6 



4 53 



1800 



8-5 



— 3 





1 



137 



224 



— — 



43-2 



24 5 



5 9- 



180 



6-8 



•0 





1 



39 



225 



— — 



43-3 



24 3 



5 18 



570 



7-6 



— 2 





2 



92 



226 



— — 



43-2 



24 4 



5 27 



57, 18, 6, 2 



5-5,4o,31,21 



— 3 





2 



80 



243 



— 13 



39-4 



24 12 



1 11 



1800 



8-8 



•0 





2 



183 



244 



— — 



39-3 



23 35 



1 51 



1821 



8-9 



+ •1 





2 



136 



245 





41-7 



23 5 



2 30 



1800 



8-7 



— 2 





1 



163 



246 



— — 



41-7 



24 35 



3 10 



1800 



90 



-1-2 









203 



247 



— — 



44-1 



23 32 



3 52 



1800 



8-5 



— 3 









149 



248 





44-2 



24 12 



4 32 



1800 



8-3 



— 5 









149 



305 



— 16 



401 



23 12 



2 20 



1800 



8-6 



— 2 





1 



124 



306 



— — 



40-1 



24 29 



2 54 



1800 



8-6 



— 2 









172 



307 



— — 



38-5 



23 50 



3 30 



1800 



8-6 



— 2 









125 



310 



— — 



41-7 



23 20 



4 54 



570 



7-5 



— 3 





1 



88 



311 



— — 



41-7 



23 20 



5 2 



180 



6-7 



— 1 





1 



62 



312 





43-3 



23 20 



5 28 



1800 



8-5 



—3 





2 



100 





9972 



were placed parallel to the meridian, so that the diffraction images appear to the 

 east and to the west of the central image, while on Aug. 12, Sept. 12 and Oct. 12 

 the grating was placed at right angles to this position. 



The plates of the same exposure time do not all show images of the same 

 intensity owing to differences in the transparency or quietness of the air or sensibility 

 of the plates. To get a measure of the efficiencj' of the different plates, I have 

 estimated on each plate the magnitude at -which the spectra of the second order 

 had a certain arbitrary' intensitj'. The efficiencies thus found are given in stellar 

 magnitudes (the zero point being arbitrary) for each plate separately in Table 1. 

 Compared with the longest exposure of 1800^ the exposures of 570^, 180^ and 57^ 

 are in the mean respectively "-975 ±™"054 (m.e.), l"i-973 ±"'060 and 3™-054 ± '"-079 

 less efficient. That is to say, that the proportion of exposure time chosen, )/lO, 

 practically corresponds to one magnitude. Hence for the plates used" in this in- 

 vestigation the exponent p in the formula of Schwarzschild 



intensif}- of photographic image = f (intensity of light X exposure time^^) 

 is found to be "80 ± "02 (m. e.). 



The mean efficienc}' of a plate is thus 8™'8-|-log (//1800), where / is the ex- 

 posure time in seconds. The deviations of the single plates from this formula are 

 given in Table 1. The mean square ™"'062 = (±™'25)'- of these deviations gives a 



