This is evidently immense progress. One of the most illustrious of German physicists, M. Dove of Berlin, gave in 1851 the name of reversion prism to the combination of two prisms placed normally one behind the other so that their corresponding faces were perpendicular. He presented this disposition as an important new discovery made by himself. He doubtless did not know that M. Porro, who deserves all the honor of this charming application, had realized it long before him.” A little later M. Porro produced what is commonly referred to as Porro’s second form, which is derived directly from annexing _A_ Fig. 115 to the corresponding half of _C_ as a single prism, the other half of _B_ being similarly annexed to the prism _C_, thus forming two sphenoid prisms, such as are shown in Fig. 118 which may be mounted separately or may have their faces cemented together to save loss of light by reflections. The sphenoid prisms have had the reputation of being much more difficult to construct than the plain right angled prisms of the other forms shown. In point of fact they are not particularly difficult to make and the best inverting eye pieces for telescopes are now constructed with sphenoid prisms like those just described. [Illustration: FIG. 118.—Porro’s Second Form.] [Illustration: FIG. 119.—Clark Prismatic Eyepiece.] This particular arrangement lends itself very readily to a fairly compact and symmetrical mounting, as is well shown in Fig. 119 which is the terrestrial prismatic eyepiece as constructed by the Alvan Clark corporation for application to various astronomical telescopes of their manufacture. A glance at the cut shows the compactness of the arrangement, which actually shortens the linear distance between objective and ocular by the amount of the path of the ray through the prisms instead of lengthening the distance as in the common terrestrial eyepiece. The field moreover is much larger than that attainable by a construction like Fig. 110, extending to something over 40°, and there is no strong tendency for the illumination or definition to fall off near the edge of the field. In the practical construction of prism field glasses the two right angled prisms are usually separated by a moderate space as in Porro’s original instruments so as to shorten the actual length of the prism telescope by folding the ray upon itself as in Fig. 120, which is a typical modern binocular of this class. [Illustration: FIG. 120.—Section of Prism Binocular.]