From ocular to objective the length was about an inch and a half. It was of 10 power and took in a field of 45 yards at a distance of 1000 yards. Here for the first time we find a prismatic inverting system of strictly modern type. And it is interesting to note that if one had wished to make a binocular “Lunette à Napoleon Troisiéme” he would inevitably have produced an instrument with enhanced stereoscopic effect like the modern prism field glass by the mere effort to dodge the observer’s nose. Somewhat earlier M. Porro had arranged his prisms in the present conventional form of Fig. 117, where two right angle prisms have their faces positioned in parallel planes, but turned around by 90° as in Fig. 114. The ray traced through this conventional system shows that exactly the same inversion occurs here as in the original Porro construction, and this form is the one which has been most commonly used for prismatic inversion and is conveniently known as Porro’s first form, it actually having been antecedent in principle and practice to the “Lunette à Napoleon Troisiéme.” The original published description of Porro’s work, translated from “Cosmos” Vol. 2, p. 222 (1852) et seq. is here annexed as it sets forth the origin of the modern prism glass in unmistakable terms. _Cosmos, Vol. 2_, p. 222.—“We have wished for some time to make known to our readers the precious advantages of the “longue-vue cornet” or télémetre of M. Porro. Ordinary spyglasses or terrestrial telescopes of small dimensions are at least 30 or 40 cm. long when extended to give distinct vision of distant objects. The length is considerably reduced by substituting for a fixed tube multiple tubes sliding into each other. But the drawing out which this substitution necessitates is a somewhat grave inconvenience; one cannot point the telescope without arranging it and losing time. For a long time we have wished it were possible to have the power of viewing distant objects, with telescopes very short and without draw. M. Porro’s “longue-vue cornet” seems to us to solve completely this difficult and important problem. Its construction rests upon an exceedingly ingenious artifice which literally folds triply the axis of the telescope and the luminous ray so that by this fact alone the length of the instrument is reduced by two-thirds.