Sentence:  A wide field of work appears open in the application of photography to meridian instruments, or to the <WSD>almucantar.</WSD> The sensitive plate should be substituted for the reticule, and the position marked by a graver attached to the tail-piece of the telescope. 
Meaning of the word marked with <WSD> tags:  An instrument for observing the heavenly bodies as they cross such a circle.