EXPLANATION OF PLATE XXVL 



Figs. 4 and 5. — Two photographs taken of sight viewed from the rear, without changing the position 

 of the camera. In fig. 4 the camera was focussed for the sight only, the distant 

 view being so much out of focus as to be quite invisible. In fig. 5 the focussing 

 was made on the distant view, and, as will be seen, the sight itself is quite indistinct, 

 but the vertical image of the + is as sharp as the distant view, showing that it is 

 formed at or near the plane of the object. 



Figs. 6 and 7. — Photographs showing sight attached to rifle. 



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