is a diagonal perspective, of which the following concise and perspicuous description is taken from Dr Hutton's Mathematical Dictionary.—ABCD (Plate XII.) 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 r s, in the end of the tube. Let P a Q be an object to be viewed, from which proceed the rays P c, a b, and Q d; these rays, being reflected by the plane mirror IK, will show the object in the direction c p, b a, d q, in the image p g, 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 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 purpose to which this glass is applied.
When the object is very near, as XY, it is viewed through a hole s y, 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 unfocuses 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. 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!
OPHRYS (See Encycl.). A new species of this plant has been lately described in the Annual Hampshire Repository, by a Fellow of the Linnean Society, in the following words:
"Stem—about 12 inches high, erect, stipulate, geniculate," Ophiucus, culate, pubescent at the upper genicles. Spike—strictly Opium, spiral, flowers spirally ascending, about 24, brightly white. Upper petal ovato-acuminate, pubescent, lightly ciliate, straight. Two middle petals oblong-recurred. Two lower petals oblong-acuminate, lightly ciliate only on the lower side near the base, projecting like elephant's tusks. Nectary, broad, recurved, ragged, biciliate. Leaves floral—carinate acuminate, ciliate reaching and pointing to the middle of the flowers. Leaves radical—five or six, about six inches long, narrow, attenuate both ways, acuminate, the lower more haste. Leaves cauline—lanceolate, alternate.
Observation.—This plant has much the habit, as well as autumnal florecescence, of Oriental spiralis, and is so perfectly spiral also, that the specific name of the other should be altered, as being no longer exclusively spiral; at the same time that a specific name should be given to this; neither of which (says the author) I shall presume to do, but shall suggest it to the Linnean Society, of which I have the honour to be a Fellow."—This ophrys flowered, for the first time, it is believed, in England, in Hampshire, October 1796.