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. In brilliancy necessarily cut down a little, not by the double total reflection, which as is well known does not lose light, but by the quadruple passage across the substance of the two prisms, the cornet in sharpness and amplification of the images can compare with the best hunting telescopes of the celebrated optician Ploessl of Vienna. M. Porro has constructed upon the same principles a marine telescope only 15 c.m. long with an objective of 40 m.m. aperture which replaces an ordinary marine glass 70 c.m. long. He has done still better,—a telescope only 30 c.m. long carries a 60 m.m. objective and can be made by turns a day and a night glass, by substituting by a simple movement of the hand and without dismounting anything, one ocular for the other. Its brilliancy and magnification of a dozen times with the night ocular, of twenty-five times with the day ocular, permits observing without difficulty the eclipses of the satellites of Jupiter.