Manchester Mejnotrs, Vol. xlvii. (1903), No. 8. 3 



exceeds in accuracy the angular measurement of ordinary 

 6 in. theodolites. Using a telescope with a focus of 160 ins. 

 for producing the photographs, angular displacements of 

 y^" could be detected, and doubtless the accuracy could 

 be greatly increased, if every precaution were employed to 

 make the transparency a direct counterpart of its negative, 

 and if two lanterns, instead of one, were used. 



One of the advantages of this method of comparing 

 photographs seems to be that the positions of hazy 

 objects, such as nebulae or comets, can be determined with 

 the same accuracy as those of stars. If the negative and 

 positive images of such an object register correctly, their 

 region is of a uniform grey tint ; the very smallest shift 

 produces a mottled appearance, and a slightly increased 

 shift shows a dark and bright nucleus in juxtaposition. 



Another very important purpose to which this 

 method can be applied is the rapid surveying of stellar 

 photographs, with a view to picking out such stars as have 

 undergone any changes of brightness or position, and I 

 believe that for this purpose the most suitable arrange- 

 ment will be not the superimposing of the negative on 

 the transparency, but the use of two lanterns with which 

 to project the images on the screen, adjusting the plates 

 till the desired agreement is attained. By slightly tilting 

 one of the plates one would get over the difficulty of 

 atmospheric refraction, and by altering the relative 

 illumination of the two plates one could readily detect 

 changes of brightness in stars and perhaps even in nebulse. 



Another purpose to which I think this method could 

 be applied is the measurement of the angular distance of 

 two close binaries. Their combined star discs, although 

 appearing circular in the eye-piece or on the photographic 

 plates, must be slightly oval, which irregularity could, I 

 think, be detected by turning one of images through an 



