STELLAR niOTOGRAPIIY. 



205 



to this time,' by giving two or more exposures to each region each star may be 

 made to indicate the region in which it is situated by a series of characters like 

 those of the Morse telegraphic alphabet. Each plate covers a region 10° square, 

 and therefore extends over 40 minutes on the equator, and more in other declina- 

 tions. Eight regions are taken on each plate in the first series. The settings 

 are made at -20°, —10°, 0°, +10°, +20°, +30°, +40°, and +50°. The region covered 

 therefore extends from —25° to +55°. One minute is devoted to cacli region. 

 The first exposure lasts as many seconds as there are degrees of declination in 

 the southern edge of the region. A break then occurs for ten seconds, and 

 for the last five regions, that is, for those north of the equator, the second exposure 

 lasts during the remainder of the minute. For the first three regions, a second 

 break of ten seconds is made, extending from the fortieth to the fiftieth second. 

 The regions covered in each case, the length of the exposures, and the appearance 

 of the trails, are as follows : 



25° to —15° "25 seconds, 5 seconds, 10 seconds, 



15° 

 - 5 

 + 5 



+ 15 



+ 25° 

 + 35° 

 + 45° 



• 



5° 15 « 15 « 10 " 



+ 5° 5 " 25 " 10 u 



+ 15° 5 " 45 



+ 25° 15 " 35 



+ 35° 25 " 25 



+ 45° 35 " 15 



+ 55° 45 " 5 



U 



a 



... 



.. 



• • * 



These plates are to be taken for every 40 minutes throughout the entire twenty- 

 four hours of right ascension. Another series is taken for every alternate twenty 

 minutes, and, to make these overlap still better, the declination is diminished for each 

 of them by 5°. Accordingly, the centres of the regions in the second series coincide 

 with the corners of the regions of the first series. Every star will therefore appear 

 on at least two regions, and if near the corner of one, it will be near the centre 

 of the other. Owing to the convergence of the meridians, some stars will appear 

 on more than two plates. The total area to be covered is about 2-5,000 square 

 degrees, and each plate will cover 800 degrees. As the series will contain 72 

 plates, the whole space Avill be covered on the average about two and a quarter 

 times. As we go north, the lines become shorter, and therefore fainter stars will 

 leave trails. At the northern limit the lines will have but little over one half 



th on the equator, and stars half a magnitude fainter will appear. The 

 most northern region will have one exposure of 45 seconds, which even at that 

 declination will give a line long enough to be readily compared with the others. 



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