EXPLANATION OF PLATE XXVI. 
Fics. 4 anp 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. 
Fries. 6 anv 7.—Photographs showing sight attached to rifle. 
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