PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY. 481 



observed. According to Bessel and Peters, this i)arallax scarcely sur- 

 passes a third of a second; according to Otto Struve and Anwers, it 

 attains a half second (0". 52). Mr. Pritchart! has photographed the 

 double star in question, during the year with four comparison stars 

 disposed symmetrically, — two in the direction of the components and 

 two in the peri)ondicular direction. The micrometric measures have 

 given him for each of the two components a jiarallax of 0".43, which, 

 indicates a distance equal to aboiit 500,000 times that of the sun, a 

 distance over which light leai)s in seven and one half years. It has been 

 necessary, however, to reject some negatives on account of accidental 

 deformations of the sensitive film occurring during development. In 

 order to free ones' self from this source of error, it is only necessary to 

 employ plates impressed with a reticule of reference, in accordance with 

 the advice of Mr. Lohse. Since last year Mr. Pritchard has simplified 

 his process of research in confining himself to observing each star dur- 

 ing five nights in each of the four periods of the year indicated by the 

 parallactic ellipse, which tlie star seems to describe in the sky ; in this 

 way he hopes to be able to determine the i)arallaxes of ten to fifteen 

 stars a year. He has commenced the work on several stars of the con- 

 stellations Oassiopea and Cygnus, whose parallaxes appear to be com- 

 prised between 0".04 and 0'M9. For Polaris Mr. Pritchard has found 

 0".07 ; that is to say, that Polaris is three millions times more distant 

 from us than the sun. 



It is admitted generally that the most brilliant stars are also the 

 nearest to us ; however, among the parallaxes which are known to be 

 sensible up to the present time many belong to stars relatively faint, 

 and nothing prevents supposing that in the number of stars which have 

 not been examined and which will soon be catalogued by photography 

 there will be found those which are even much nearer to our solar sys- 

 tem. However, they could not delay being disclosed, since the sinqtle 

 microscopic inspection of the same group photographed at six months' 

 inlerval would suffice to reveal sensible parallactic displacements bow- 

 ever small. 



The direct micrometric measures of groups of stars reveal to us only 

 displacements in the direction perpendicular to the visual ray; the spec- 

 troscope alone can make us acquainted with movements which take place 

 in a direction the samc! as the visual ray. For the color of light which 

 comes to us fiom a star, is slightly modified by the velocity with which 

 the star ap])roaches or removes itself from us, and it follows that rays 

 of the spectrum are deviated a little towards the right or towards the 

 left. (It is for the same reason that the locomotive-whistle seems to us 

 sharper in pitch when the train is approa(;hing than when it is receding.) 

 It is i)0ssible by this means to estimate the velocity of translation of a 

 certain number of bright stars whose spectra are not too difficult to 

 observe. However, the eye is fatigued in comparing with the motion- 

 Jess rays of an artificial spectrum the always trembling lines of tli^ stellar 

 H. Mis. 224 31 



