The Telescope

by Louis Bell

Fig. 116.—Lunette à Napoleon Troisiéme.

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.

Fig. 117.—Porro’s First Form of Prisms.

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.

Let us try to give an idea of this construction: Behind the telescope objective M. Porro places a rectangular isosceles prism of which the hypothenuse is perpendicular to the optic axis. The luminous rays from the object fall upon the rectangular faces of this prism, are twice totally reflected, and return upon themselves parallel to their original direction: half way to the point where they would form the image of the object, they are arrested by a second prism entirely similar to the first, which returns them to their original direction and sends them to the eyepiece through which we observe the real image. If the rectangular faces of the second prism were parallel to the faces of the first, this real image would be inverted—the telescope would be an astronomical and not a terrestrial telescope. But M. Porro being an optician eminently dextrous, well divined that to effect the reinversion it sufficed to place the rectangular faces of the second prism perpendicular to the corresponding faces of the first by turning them a quarter revolution upon themselves.

In effect, a quarter revolution of a reflecting surface is a half revolution for the image, and a half revolution of the image evidently carries the bottom to the top and the right to the left, effecting a complete inversion. As the image is thus redressed independently of the eyepiece one can evidently view it with a simple two-lens ocular which decreases still further the length of the telescope so that it is finally reduced to about a quarter of that of a telescope of equal magnifying power, field and clearness.

The new telescope is then a true pocket telescope even with a magnifying power of 10 or 15. Its dimensions in length and bulk are those of a field glass usually magnifying only 4 to 6 times. The more draws, the more bother,—it here suffices to turn a little thumbscrew to find in an instant the point of sharpest vision.