C. S. Sellack—Photographing of Southern Star-clusters. 17 
found the increase of the area of the image proportional to time. 
atmosphere produces a strong scintillating motion, the images 
of bright stars become very large by long exposures, and faint 
stars do not produce any impression. In great zenith-distances 
another obstacle combines with oe increased scintillation, the 
strong absorption of chemical rays by the atmosphere. 
The increase of refraction depending on sonith- distance, for 
an exposure of eight minutes, is computed in wa ascension 
and declination for declinations from +20° to — 80° for Cor- 
doba, southern latitude 31° 15’ 30,” ae 20° , and 
barometer 725 ™™ 
Differences of refraction. 
are +20" +10° 0° —10° =—20° + =~380° a —50° 
angle, @ O a ae 6 8 ao 
Tz a” ie a” Be, ” | a Be a” ” ” | a” by we A. 72 a” 7 ” a” 
0 . 0 0-02/0-131|0 02/0°14/|0-00|0-07 
| plea pl a He eee ete ic aaiaat en ates> 
: *|(0-11]0-67)|0-11|0-57||0-1 14.0°56 0°23/0°62)/0°51/0°77 0°68) /0°LT/0°39 
13"-15"||0°34| 1-18||0°33|0-97| (0°38 0-90||0°55|0°93 |0°97|0-92 161 0-29||1-08 0-641|0-46/0- 
28°~30"||1°51|2-63)|1-30/1-95]/1-29 1-58|/1-23)1-50||1-69/0-94)|1-88/0°36))1°65)0-29 1:17/0°63 
43°65" 2°75 2°37T||2°33/2-01||2°50|1°17||2°39 0°59|/2-19/0-04||1-79|0-38 
ot he the component of the refraction in right ascension can 
be reduced to less than 0-1, by retarding the clock by the 
mean amount, and for intervals of 8™ within the given = 
(with exception of the zenith-stars near culmination). The c 
rections of the clock per hour-are : 
Interv: 
of our 
Ne og +20°}+10°| 0° |—10°/—20°|—30°|—40°|—50°|—60°) —80 
8. 8. M % 8. 8. 8, + 
0°— 2°!| 0-00} 0:00] 0-00| 0:00| 0-01} 0:33} 0-01| 0-00) 0°00, 0-00 
7 9°l| 0-06) 0°05| 0-07| 0-12| 0:27| 0°59| 0-41| 0-13 0-08 | 0-06 
13°-15°|| 0-18 | 0:17| 0°19! 0:28| 0°52| 0°93| 0°70) 0°36) 0°22) 0-15 
25-80" 0°80 0°66] 0°64! 0°63) 0°90! 1°08) 1°08| 0°91) 0°74) 0°66 
43°-45°|| 1:37| 1:18] 1°43) 1°38} 1-43| 1°39) 1°32| 1°38 
.The component of refraction in declination cannot be elimin- 
ated, But the 
table shows that a double exposure of eight 
minutes can be made for all stars in. wick the motion of the 1m- 
age of the central star is less than 0°’2 or 0°004™™ on the photo- 
inwore plate. The field comprehends a circle of 80’ —) 
“lea the difference of the motion of stars near the border, and 
. the central one, is always less than 0.”1. The favorable 
me for taking photographs is for north culminating stars near 
si imination, or stars culminating near the zenith at some 
istance from culmination, for south eens stars near 
Am. Jour. Sct.—Turp SERIES, Vou. VI, No. 31.—Joury, 1 
2 
