480 PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY. 



a cause the inovement of translation of the solar system, aud which per- 

 mit the (letermiuation of its velocity aud directiou.* 



Sometimes the progression, instead of being uniform and continuous, 

 is found to be affected with periodic inequalities which reveal the ex- 

 istence of an annual parallax, that is to say, a sensible effect produced 

 by the change of position of the observer when the earth passes from 

 one extremity to the other of its orbit; the oscillation in the apparent 

 l)lace of a star, which results from it permits the calculation of the dis- 

 tance which separates it from our system. Or indeed the inequality 

 presents a longer period, and the successive positions of the star per- 

 mit the discovery of an orbit which it describes around a neighboring 

 center of attraction. We deal here with a physical couple; oi)tical 

 couples, where the nearness of position is only the effect of perspective, 

 present independent proper motions. 



Researches of this sort will without doubt be facilitated by the 

 application of photograph} , for the determination of the relative posi- 

 tions upon the negative will be infinitely more convenient than in the 

 field of the telescope, especially when a comparison is desirable between 

 stars of very different brilliancy. In certain cases, indeed, photography 

 offers the only means of obtaining precise measures; how would we at- 

 tempt directly the measurement of the distances aud position angles in 

 a mass of stars such as the cluster in Hercules"? Upon the negative 

 this cluster forms a small diffuse spot 2 to 3 millimeters in size ; on ex- 

 amining it with a lens we distingush several hundred stars dispersed 

 around a nucleus of a pulverulent appearance, which we may proceed to 

 without doubt resolve in its turn into a multitude of lumiuous points. 

 We have never attempted to design these groupings, still less to make 

 direct luicrometric measures, the eye being dazzled, says Mr. Moucliez, 

 by what appears in the eye- piece as a mass of innumerable and brilliant 

 grains of dust; but the negative placed under the microscope will per- 

 mit us to draw without difticulty the exact chart of this wonderful corner 

 in the sky. ■ In transmitting it to posterity we will give to our descendants 

 the means of verifying the evolutions, which without doubt are slowly 

 accomplished in the bosom of this agglomeration of suns. 



The research of the annual parallax, which permits us to measure the 

 distance of the stars by taking for a base of operations the diameter 

 of the terrestrial orbit constitutes one of the most delicate problems of 

 modern astronomy, for the displacements, which it concerns us to verify, 

 never surpass a few tenths of a second, and are oftener masked errors 

 of observation, whence the irritating discordance of successive de- 

 terminations of the same parallax effected by astronomers equally skillful 

 with the most perfect instruments. Will photography be more fortu- 

 nate 1 Mr. Pritchard at Oxford has attempted to utilize 61 Cygni in the 

 first place for the verification of the parallax of a double star very often 



* See iu the Bevue des Deux Mondcs of October 1, 1875, " The Progress of Stellar As- 

 tro uoiiiy." 



