Manchester Memoirs, Vol. xlvii. (1903), No. 8. 



VIII. Parallax Determinations by Photography. 



By C. E. Stromeyer, M.Inst.C.E. 



Received and read February ^rd, iQOj. 



When announcing my intention to read a paper on 

 this subject, I was not aware that our member, Mr. 

 Thorp, had brought the matter before the British 

 Astronomical Association, and that Mr. Pickering was 

 employing the same method or a similar one for certain 

 purposes at the Harvard Observatory. My application 

 of the method to land surveying is I think new, and will, 

 I believe, interest engineers. 



The method consists in superposing the image of a 

 negative photograph taken at one period or position and 

 the image of a transparency taken at another period or 

 position. If the two images were identical then the 

 result of the combination would be a field of view of 

 uniform tint ; if, however, there are changes of position, 

 then the two images will register only locally and the 

 relative displacements of two objects can be measured 

 micrometrically by noting the amount of displacement of 

 the two images. 



It is not my intention to deal with the reduction of 

 these measurements, except to mention that the most 

 convenient conditions for taking surveying photographs is 

 to arrange to have the two negatives on one plane. Then, 

 as in the present instrument, one negative and one 

 positive can be placed in contact, or these plates can be 

 placed in two separate lanterns, and their images be 

 thrown on one screen or into an eye-piece. In either 



April 23rd, igoj. 



