﻿372 B. A. Gould — Determinations of Stellar Positions. 



of measurements of the same stars, derived from various plates, 

 will at once make manifest the degree of confidence to which 

 the several values and their mean are respectively entitled. 



A far more serious obstacle to accuracy is presented by the 

 difficulty of obtaining absolute^ round images. Irregularity 

 of form in the dots formed by the stellar impressions is almost 

 incompatible with precision of measurement; and, as the time 

 of exposure must often be long, the chief problem was, not so 

 much to obtain the images as to insure uniformity of motion in 

 the telescope during the period of exposure. Not that the pho- 

 tographic processes were not troublesome enough before the 

 introduction of the dry -plate processes, for very great care and 

 numerous precautions were often necessary to prevent the 

 plates from drying too fast ; but far the greatest difficulty con- 

 sisted in obtaining sufficient precision in the clockwork and 

 equatorial motion of the telescope. 



It may easily be imagined how great was my desire, when 

 leaving home for South America, to extend this new method 

 of observation to the Southern hemisphere. But the obstacles 

 encountered in the endeavor cannot be easily imagined. Upon 

 these I will not enlarge here further than by saying that, in 

 Cordoba, also, the attainment of circular dots for the star-images 

 offered incomparably the greatest of all the difficulties of a prac- 

 tical character. The time of exposure was limited by the max- 

 imum size allowable for the large stars ; and, previous to 1878, 

 also by the drying of the plate, although exposures of twenty 

 minutes were not unusual. Nevertheless, by dint of specially 

 constructed governors and regulators, and by ceaseless atten- 

 tion, we did succeed in obtaining impressions which, to the 

 unaided eye, appear absolutely round. 



This necessity of long-continued and minute uniformity in 

 the motion of the telescope, is, of course, largely diminished by 

 the employment of instruments of large aperture, inasmuch as 

 the necessary time of exposure is diminished in the same ratio 

 in which the amount of light is increased. It is yet further and 

 most notably diminished by the manifold greater sensitiveness 

 of the dry gelatine plates. But notwithstanding all this, the 

 attainment of round images, while almost indispensable for 

 giving to stellar photography that increased accuracy to which 

 it may lay claim as a means of research in practical astronomy, 

 still demands especial care and precaution. 



The Argentine Government cordially afforded every assist- 

 ance which I deemed it proper to ask for these investigations. 

 And although the chief energies of the Cordoba Observatory 

 were absorbed by those investigations for which the institution 

 was established, I had the satisfaction of obtaining a sufficient 

 number of stellar photographs to occupy not only my own life- 



