horizon-glass

(h6-ri’zon-glas), n. In astron., the small plane mirror which is firmly attached to the frame of a quadrant or sextant, and has one half silvered. In measuring an altitude of the sun the observer looks directly through its transparent half toward the horizon at the point directly under the sun. Formerly two horizon-gla es were often used, one the front gla as above described, the other, the back gla , so placed that the observer looked through it to the point of the horizon opposite to that under the sun: this gla had simply a narrow unsilvered strip acro its middle.