STELLAK PHOTOGRAPHY. 



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 also of some faint 

 close polars, is given in Table III. The successive columns give the number from 

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

 declination for 1885, its brightness to the eye, and the corresponding brightness of 

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

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

 the Proceedings of the American Association, XXXIII. 1, except )8 Ursce Ilinom, 

 which is taken from the Harvard Annals, XIV. 200, and <t Odantis, which is taken 

 from the Uranometria Argentina, p. 131. 



TABLE III. 



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 

 effect on the result, except in obscuring details, unless the surface is very small. 



Using the same notation as before, / = ^ 



r 



't T- 



Accordingly, equally faint surfaces 



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 



