OPERA GLASS, is a diagonal perspective, of which the following concise and perspicuous description is taken from Dr Hutton's Mathematical Dictionary.—ABCD (Plate XLI.) represents a tube about four 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 eye-glass NO, whence they emerge parallel to the eye at the hole rs, in the end of the tube. Let P a Q be an object to be viewed, from which proceed the rays P e, a b, and Q d: these rays, being reflected by the plane mirror IK, will shew the object in the direction c p, b a, d q, in the image p q, equal to the object P Q, 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 three inches, and that of NO one inch, the distance between them will be but two inches, and the object will be magnified three 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 cleaning 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. It is chiefly used in play-houses; and hence its name: but we have seen it most indecently employed by those who should have set a better example, even in a cathedral church!