A decidedly better form of positive ocular is the modern orthoscopic as made by Steinheil and Zeiss, Fig. 105_a_. It consists of a triple achromatic field lens, a dense flint between two crowns, with a plano-convex eye lens of much shorter focus (⅓ to ½) almost in contact on its convex side. The field triplet is heavily over-corrected for color, the front focal plane is nearly ½ F ahead of the front vertex of the field lens, and the eye distance is notably greater than in the Kellner. The field is above 40°, beautifully flat, sharp, and orthoscopic, free of troublesome ghosts. On the whole the writer is inclined to rate it as the best of two-lens oculars. There should also here be mentioned a very useful long relief ocular, often used for artillery sights, and shown in Fig. 105_b_. It consists like Fig. 104_a_, of a pair of achromatic lenses, but they are placed with the crowns almost in contact and are frequently used with a simple plano convex field lens of much longer focus, to render the combination more fully orthoscopic. The field, especially with the field lens, is wide, quite 40° as apparent angle for the whole instrument, and the eye distance is roughly equal to the focal length. It is a form of ocular that might be very advantageously used in finders, where one often has to assume uncomfortable angles of view, and long relief is valuable. [Illustration: _a_ _b_ FIG. 105.—Orthoscopic and Long Relief Oculars.] Whatever the apparent angular field of an ocular may be, the real angular field of view is obtained by dividing the apparent field by the magnifying power. Thus the author’s big Kellner, just mentioned, gives a power of 20 with the objective for which it was designed, hence a real field of 2½°, while a second, power 65, gives a real field of hardly 0°40′, the apparent field in this case being a trifle over 40°. There is no escaping this relation, so that high power always implies small field. The limit of apparent field is due to increasing errors away from the axis, strong curvature of the field, and particularly astigmatism in the outer zones. The eye itself can take in only about 40° so that more than this, while attainable, can only be utilized by peering around the marginal field. For low powers the usable field is helped out by the accommodation of the eye, but in oculars of short focus the curvature of field is the limiting factor. The radius of curvature of the image is, in a single lens approximately 3/2 F, and in the common two lens forms about ¾ F. In considering this matter Conrady has shown (M. N. _78_ 445) that for a total field of 40° the sharpness of field fails at a focal length of about 1 inch for normal power of accommodation. The best achromatic combinations reduce this limit to about ½ inch.