MOUNT WILSON OBSERVATORY. 



231 



by Mr. Seares, assisted by Messrs. Hubble and Duncan. Equal ex- 

 posures were made simultaneously with the two telescopes to the same 

 field, using plates from the same box, which were afterwards developed 

 in the same tray. To eliminate the influence of accidental variations 

 in the sensitiveness of the plates, two or more photographs of the same 

 field were made in each series. 



The results for limiting magnitude are shown in the accompanying 

 tables. All the photographs were on Seed 30 plates, exposed to Selected 

 Area 87, for which the photographic magnitudes on the normal scale, 

 referred to the international zero-point, are well determined. The 



Limiting magnitudes, 100-inch and 60-inch reflectors. 



May 22: Not representative of the performance of the 100-inch. Images very large as com- 

 pared with those of August 14 and 15. Mirror astigmatic; figure of 60-inch excellent. 



August 14: A fair test of both instruments. Note consistencj- of limits. 



August 15: Gain for first pair too large because of poor images on 60-inch plate. The second 

 pair is a fair comparison. 



limit was obtained by selecting 10 or 12 stars on each plate, just faintly 

 visible, and estunating in tenths of a magnitude the interv-al separating 

 their images from the limit. The kno^vm magnitude, plus the estimated 

 interval, thus gives for each star a value of the limit. The tabular 

 values are the mean results for individual plates. The probable error 

 arising from the estimates is 0.03 or 0.04 magnitude. 



For a 2™ exposure on Seed 30 plates we may accept as representative: 



A similar plan was followed in the tests of resolution, the fields in 

 this case being the central part of the extended nebulosity M 8, and 

 the planetary nebula N. G. C. 7009, which is especially rich in fine, deli- 

 cate detail. One pair of plates of the Ring Nebula in Lyra was also 

 made, but the 100-inch photograph is defective through faulty guiding. 



