STELLAR PH OTOG RAPHY. 



191 



All of these formulas are of course approximate, and, if used for intervals of time 

 in which the change in right ascension is very great, the neglected terms may have 

 to be considered. 



The relative brightness of the trails of the brighter stars, and al-o of some faint 



close polars, is given in Table III 



The 



giv 



the 



umber from 



the Harvard Photometry, the usual designation of the star, its right ascension and 



declination for 1885. its br 



the eye, and the 



iponding brightness of 



the photographic trail. The first eight magnitudes in the last column but one wen 

 taken from the Harvard Observatory Annals. XIV. 406. The others are Liken iron 

 the Proceedings of the American Association, XXXIII. 1, except /3 Ursce Mlnom 



which is 



from the Harvard Annals, XIV. 200, and a- Octant is. which 



from the Uranometria Argentina, p. 131 



TABLE III. 



H.P. 



1275 



2400 



932 



93G 



3147 



213 



2500 



3077 



1292 



3426 



Desig. 



a Canis Majoris 



a Argus . . . 

 a Centauri . . 



a Bootis . . • 



a Aurio'OB . . . 



p Orionis - . • 

 a Lyras . • . . 

 a Ursae Minoris 



j3 Ursae Minorifl 



8 Ursse Minoris 

 51 H. Cephei . . 

 X Ursae Minoris 

 DM +89° 3 . . 

 a Octantis . . . 

 DM +89° 37 . 



R. A. 1885. 



m. 8. 



6 40.1 



6 21.4 



14 31.8 



14 10.4 



18 33.0 

 16.6 

 14 51.0 



6 46.7 



19 37.9 



2 38.8 



18 33.5 



18 24.0 



Dec. 1885. 



16 34 



oo 



+ 19 



+ 45 



+ 38 

 + 88 42 

 + 74 38 

 + 86 37 

 + 87 13 

 + 88 58 

 + 89 38 

 -89 16 

 + 89 54 



Magn. 



10.5 



Trail 



The conditions needed to photograph a faintly illuminated surface, as a nebula, 

 are quite different. The four sources of error noted on page 187 will here have no 



the surface is very small. 



b ./, m . Accordingly, equally faint surfaces 



in obscuring details, unless 



/ 



dHT' 



effect on the result, except 



Using the same notation as before, I 

 may be photographed by similar lenses, whatever their size, except that a large lens 

 has a slight disadvantage from the greater absorption due to the increased thickness. 

 When, however, detail is to be shown, the advantage of the large scale of the images 



i 



