In 1850 a French engineer, Porro, discovered a combination of prisms which, when inserted between the objective and the eyepiece of an astronomical telescope, showed the image erect or in its natural position, while the same telescope without the prisms showed the image inverted. Practical use of this discovery was not made for many years after. These prisms served a twofold purpose, viz, showing the image of the object looked at in its natural position instead of reversed, and second, the shortening of the telescope by twice turning the ray of light upon itself. Each tube of the prism field glass contains two of these double-reflecting prisms. The ray of light passing through the object glass enters the first prism in such a manner as to be twice totally reflected, each time at an angle of 90°, thus emerging parallel to the entering ray, but in the opposite direction. It is thus caught by the second prism and is similarly reflected and sent on its original direction without change except in one very important point, viz, the image of the object observed, which, without the intervention of the prism, would be upside down, is now erect, and will be magnified by the simple astronomical eyepiece just as the stars and planets are magnified in large telescopes. The field of view of the Porro prism glass is considerably larger than that of the ordinary field glass. It decreases about 12½ per cent with each magnifying power, a number 6-power glass giving a linear view of 118 feet in a thousand, while in a number 10 glass the field is but 70 linear feet. This is explained as follows: The rays of light emerging from the ocular of the Galilean telescope are divergent and cover an area much greater than the size of the pupil of the eye. As all rays falling outside the pupil of the eye are lost, but a small field of view can be seen, as in looking through an ordinary cone from the larger end. The prism glasses are constructed on the opposite principle. The rays of light gathered by the objective emerge from the eyepiece in a converging pencil of light small enough to enter the pupil of the eye, thus giving a larger field of view; theoretically, nine times the area given by the old-style instrument of the same power. With these advantages, however, the Porro prism glass has not been found in all respects satisfactory for field service. With a clear atmosphere and the object which is being viewed well illuminated, it is distinctly superior to the Galilean field-type glass in respect to light, power, and definition. The prisms having once been deranged, however slightly, satisfactory use of the glass can not be had until the prisms have been readjusted, and until very recently it was impracticable to have this done elsewhere than at the place of manufacture of the glass. [Illustration: FIG. 29.--Porro prism.] FIELD GLASSES.