22S Sir J. J. Thomson on 



the bottom third of the plate comes opposite the opening, 

 taking a photograph, then lowering the plate still further until 

 the middle third comes opposite the opening, taking another 



Fig. 2. 



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W 



photograph, then lowering the plate until the top third comes 

 opposite the opening, and again taking a photograph, three 

 photographs can be taken without opening the tube. This is 

 very convenient, for the deflexions of the different positive 

 rays vary so much that it is difficult to measure them when 



