OPAKE

, not translucent, nor transoarent, or not admitting a free passage to the rays of light.

OPEN Flank, in Fortification, is that part of the flank which is covered by the orillon or shoulder.

OPENING of the Trenches, is the first breaking of ground by the besiegers, in order to carry on their approaches towards a place.

Opening of Gates, in Astrology, is when one planet separates from another, and presently applies to a third, bearing rule in a sign opposite to that ruled by the planet with which it was before joined.

OPERA-Glass, in Optics, is so called from its use in play-houses, and sometimes a Diagonal Perspective, from its construction, which is as follows. ABCD (fig. 5, pl. xvii) represents a tube about 4 inches long; in each side of which there is a hole EF and GH, exactly against the middle of a plane mirror IK, which reflects the rays falling upon it to the convex glass LM; through which they are refracted to the concave eyeglass NO, whence they emerge parallel to the eye at the hole rs, in the end of the tube. Let PaQ be an object to be viewed, from which proceed the rays Pc, ab, and Qd: these rays, being reflected by the plane mirror IK, will shewthe object in the direction cp, ba, dq, in the image pq, equal to the object PQ, and as far behind the mirror as the object is before it: the mirror being placed so as to make an angle of 45 degrees with the sides of the tube. And as, in viewing near objects, it is not necessary to magnify them, the focal distances of both the glasses may be nearly equal; or if that of LM be 3 inches, and that of NO on e inch, the distance between them will be but 2 inches, and the object will be magnified 3 times, being sufficient for the purposes to which this glass is applied.

When the object is very near, as XY, it is viewed through a hole xy, at the other end of the tube AB, without an eye glass; the upper part of the mirror being polished for that purpose, as well as the under. The tube unscrews near the object-glass LM, for taking out and cleansing the glasses and mirror. The position of the object will be erect through the concave eye-glass.

The peculiar artifice of this glass is to view a person at a small distance, so that no one shall know who is observed; for the instrument points to a different object from that which is viewed; and as there is a hole on each side, it is impossible to know on which hand the object is situated, which you are viewing.