If the objectives are easily removable unscrew one of them to obtain a clear idea as to the actual size of the prisms.[19] Look out, too, for ghosts of bright stars. [19] There are binoculars on the market which are to outward appearance prism glasses, but which are really ordinary opera glasses mounted with intent to deceive, sometimes bearing a slight variation on the name of some well known maker. The objectives of prism glasses usually run from ¾ inch to 1½ inch in diameter, and the powers from 6 to 12. The bigger the objectives the better, provided the prisms are of ample size, while higher power than 6 or 8 is generally unnecessary and disadvantageous. Occasional glasses of magnifying power 12 to 20 or more are to be found but such powers are inconveniently great for an instrument used without support. Do not forget that a first class monocular prism glass is extremely convenient and satisfactory in use, to say nothing of being considerably less in price than the instrument for two eyes. A monocular prism glass, by the way, makes an admirable finder when fitted with coarse cross lines in the eyepiece. It is particularly well suited to small telescopes without circles. [Illustration: FIG. 121.—Binocular with Extreme Stereoscopic Effect.] Numerous modifications of Porro’s inverting prisms have been made adapting them to different specific purposes. Of these a single familiar example will suffice as showing the way in which the Porro prism system can be treated by mere rearrangement of the prismatic elements. In Fig. 121 is shown a special Zeiss binocular capable of extreme stereoscopic effect. It is formed of two Porro prism telescopes with the rays brought into the objectives at right angles to the axis of the instrument by a right angled prism external to the objective. The apertures of these prisms appear pointing forward in the cut. As shown they are in a position of maximum stereoscopic effect. Being hinged the tubes can be swung up from the horizontal position, in which latter the objectives are separated by something like eight times the interocular distance. The stereoscopic effect with the tubes horizontal is of course greatly exaggerated so that it enables one to form a fair judgment as to the relative position of somewhat distant objects, a feature useful in locating shell bursts. The optical structure of one of the pair of telescopes is shown in Fig. 122 in which the course of the entering ray can be traced through the exterior prism of the objective and the remainder of the reversing train and thence through the eyepiece. This prism erecting system is obviously derived from the “Lunette à Napoleon Troisiéme” by bringing down the prism _B_ upon the corresponding half _A_ and cementing it thereto, meanwhile placing the objective immediately under _A_.