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the lower edge of the index glass itself, by grinding and po- 

 lishing this edge perpendicular to the plane of the glass. The 

 adjustment would be thus rendered admirably easy and certain, 

 if the edge of the glass be formed perfectly plane and truly 

 at right angles to it's face ; were it not that this edge is neces- 

 sarily ,so narrow, as not to afford a sufficient field of view to 

 the observer, for distinguishing the object by which the ad- 

 justment is to be made : for the rays fall on the edge of the 

 mirror so obliquely (making an angle with the plane of the 

 edge, of no more than about 21 or 22 degrees, and forming oii 

 the back horizon glass an image equal in breadth, on its 

 oblique surface, to the edge), that if the index glass were so 

 great as half an inch in thickness, its edge would subtend at 

 the eye near the horizon glass an optic angle of about 85 mi- 

 nutes ; and if its thickness be, as usual, 1th of an inch, it would 

 take in a field of only about 20 minutes; which is too small 

 to distinguish with ease the terrestrial objects to be viewed, 

 though it would serve with difficulty for adjustment by the 

 contact of the edges of the direct and reflected imaires of 

 the sun or moon : this however it would do with all facility, 

 if the thicknesses of the index and horizon glasses were such, 

 and so proportioned to each other, that the image of the 

 former might be reflected from the fore and back surfaces 

 of the back horizon glass, single, so as to form one image of 

 double breadth, by the double reflection : for which purpose 

 the buck horizon glass must be very thin, and the index glass 

 too thick; as otherwise the image from the under surface of 



