1890.] Photography for determining Variability in Stars. 137 



January 23, 1890. 



Sir Gr. GABRIEL STOKES, Bart., President, in the Chair. 



The President announced that at the next meeting a Member of 

 Council would be balloted for in place of the Rev. S. J. Perry, 

 deceased. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. " On a Photographic Method for determining Variability in 

 Stars." By Isaac Roberts, F.R.A.S. Communicated by 

 Professor J. Norman Lockyer, F.R.S. Received January 

 14, 1890. 



(Abstract.) 



Some of the uncertainties which necessarily attend the determina- 

 tion of variability in the brightness of stars by eye observations are 

 removed by the application of photographic methods, and particularly 

 by that of giving two or more exposures of the same photographic 

 plate to a given sky space, with intervals of days or weeks between 

 each exposure. 



In this way any errors caused by atmospheric, actinic, or chemical 

 changes, together with those due to personal bias, are eliminated, 

 and the study of stellar variability can be pursued under conditions 

 that admit of the necessary exactitude. 



As an illustration of the applicability of this dual photographic 

 method, the enlargement on paper from, the negative, which accom- 

 panies the full paper, shows the results obtained by two exposures of 

 the same plate to the sky in the region of the great nebula in Orion. 

 The first exposure was of two hours' duration on the 29th January, 

 and the second of two and a half hours on the 3rd February, 1889. 

 The stellar images formed during the two exposures are 0*0122 of an 

 inch apart, measured from centre to centre, and are therefore com- 

 parable with each other in the field of a microscope. When the images 

 are examined in the manner thus indicated and their diameters also 

 measured by means of a suitably made eye-piece micrometer, it is 

 found that at least ten of the photographed stars, the magnitudes of 

 which are estimated to range between the 7th and 15th, have 

 changed to a considerable extent in the short interval of five days. 



