34 REPORT — 1891. 



of Professor Kapteyn, a preliminary photograpliic survey of the Southern 



VipOTTOTlg 



With an exposure sufficiently long for the faintest stars to impress 

 themselves upon the plate, the accumulating action still goes on for the 

 brighter stars, producing a great enlargement of their images from opti- 

 cal and photographic causes. The question has occupied the attention of 

 many astronomers whether it is possible to find a law connecting the 

 diameters of these more or less over-exposed images with the relative 

 brightness of the stars themselves. The answer will come out undoubt- 

 edly in the afiarmative, though at present the empirical formulae which 

 have been suggested for this purpose differ from each other. Captain 

 Abney proposes to measure the total photographic action, including 

 density as well as size, by the obstruction which the stellar image offers 

 to light. 



A further question follows as to the relation which the photographic 

 magnitudes of stars bear to those determined by eye. Visual magnitudes 

 are the physiological expression of the eye's integration of that part of 

 the star's light which extends from the red to the blue. Photographic 

 magnitudes represent the plate's integration of another part of the star's 

 light — namely, from a little below where the power of the eye leaves off 

 in the blue, to where the light is cut off by the glass, or is greatly re- 

 duced by want of proper corrections when a refracting telescope is used. 

 It is obvious that the two records are taken by different methods in 

 dissimilar units of different parts of the star's light. In the case of cer- 

 tain coloured stars the photographic brightness is very different from the 

 visual brightness ; but in all stars changes, especially of a temporary cha- 

 racter, may occur in the photographic or the visual region, unaccompanied 

 by similar changes in the other part of the spectrum. For these reasons 

 it would seem desirable that the two sets of magnitudes should be tabu- 

 lated independently, and be regarded as supplementary of each other. 



The determination of the distances of the fixed stars from the small 

 apparent shift of their positions when viewed from widely separated posi- 

 tions of the earth in its orbit is one of the most refined operations of the 

 observatory. The great precision with which this minute angulai 

 quantity, a fraction of a second of arc only, has to be measured, is so deli- 

 cate an operation with the ordinary micrometer, though, indeed, it was with 

 this instrument that the classical observations of Sir Robert Ball were 

 made, that a special instrument, in which the measures are made by 

 moving the two halves of a divided object glass, known as a heliometer, 

 has been pressed into this service, and quite recently, in the skilful hands 

 of Dr. Gill and Dr. Elkin, has largely increased our knowledge in this 

 direction. 



It is obvious that photography might be here of great service, if we 

 could rely upon measurements of photographs of the same stars taken at 

 suitable intervals of time. Professor Pritchard, to whom is due the 



