MOUNT WILSON SOLAR OBSERVATORY. 247 



parisons, the other intercomparisons of certain areas with the North 

 Pole. Each area is compared with the preceding and following area 

 of the same zone, thus connecting adjacent regions by two plates, each 

 with a symanetrical arrangement of exposures on the areas compared. 

 The complete series for a zone permits a reduction of all the relative 

 magnitudes of that zone to a common zero-point. For the final 

 reduction to the International zero-point defined by the Polar Stand- 

 ards, six areas in each zone, at intervals of 4 hours in right ascension, are 

 compared in duplicate directly with the Pole. The distribution of the 

 polar comparisons in right ascension facilitates the adjustment of the 

 closing error of the zones. 



Of the 430 scale plates thus far obtained, 301 have been completely 

 measured and 56 have been measured once. The reductions are 

 complete up to and including the relative magnitudes for 70 of the 

 areas, and for each of these a series of standards has been forwarded 

 to Professor Kapteyn to be used in the reduction of the Selected Area 

 Durchmusterung plates. 



It is now clear that the amount of material ultimately to be accumu- 

 lated will be far in excess of the original estimates, owing to the 

 unexpected richness of the fields in and near the Milky Way. For the 

 70 areas whose relative magnitudes are complete the total is over 23,000 

 stars. A provisional investigation of the distribution with respect to 

 the Galaxy indicates a much greater galactic condensation than that 

 found by Chapman and Melotte. Although the totals are determined 

 by the limiting magnitude, which varies from plate to plate, it is 

 unhkely that the values for precisely determined intervals of brightness 

 will modify the result. 



Photovisual Magnitudes for the Selected Aheas. 



Photographic magnitudes can not be utilized to their full value unless 

 photovisual results are also available. Mr. Scares and Mr. Shapley 

 have accordingly undertaken the determination of photovisual stand- 

 ards for 37 of the Selected Areas — the area near the Pole and the six 

 others in each of the six 15° zones which, in the program for photo- 

 graphic magnitudes, have been compared directly with the Pole. 

 The plan is similar to that for the photographic investigation. The 

 zonal and polar comparisons are the same in the two cases, but the 

 observations for magnitude scale have been modified because of the 

 longer exposures necessary with the isochromatic plate and yellow 

 filter. Three photographs are to be made for each area: one of six 

 5-minute exposures with apertures of 60, 32, and 14 inches; and two 

 others, each including exposures of 60 minutes and 5 minutes with 

 the full aperture. 



The absolute scale, to the fourteenth or fifteenth magnitude, will be 

 estabUshed by the plates of reduced aperture. The results from the 



