QUADRANT. 



horizon glass, and is also adjusted to be perpendicular to the plane 

 of the quadrant, the upper part of this glass is unsilvered, so that 

 the eye, applied at the eye-hole D, -may look through it. The 

 index A B, carries, what is called a vernier, which subdivides 

 the graduations on the limb of the instrument E F, into smaller 

 portions, usually into minutes. When the index is s6t to 0, and 

 the eye applied at D, the observer will perceive, if he looks through 

 the horizon glass at the horizon, that the portion of the horizon 

 glass which, being silvered, would prevent his looking through, 

 will, -nevertheless, show the horizon in it almost as plain as if it 

 was transparent, it being reflected on to it by the index glass A, 

 and then again reflected to the eye, thus, Fig. 1, A is the index 



(Fig. I). (Fig. 2). 



glass, its back being towards the eye, and C the horizon glass, and 

 D E the horizon, Seen almost as plain wthe silvered portion of C, 

 as through the transparent part. If the glasses are all rightly ad- 

 justed, then, even if the position of the quadrant be altered, as in 

 Fig. 2, the line of the horizon will still be unbroken, but move the 

 index ever so little towards 1, or 2, and immediately the reflected 

 image of the horizon will sink down, as shown in this diagram, 



a space equal to that moved over by the index, and if a star should 

 happen to be just so many degrees, or parts of a degree, above 



