The inter-pupillary distance is generally a scant 2½ inches so that the clear aperture of one of the objectives of a field glass is rarely carried up to 2 inches. The best field glasses have each objective a triple cemented lens, and the concave lenses also triplets, the arrangement of parts being that shown in Fig. 110. Glasses of this sort rarely have a magnifying power above 5. In selecting a field glass with the idea of using it on the sky try it on a bright star, real or artificial, and if the image with careful focussing does not pull down to a pretty small and uniform point take no further interest in the instrument. [Illustration: FIG. 110.—Optical Parts of Field Glass.] The advantage of a binocular instrument is popularly much exaggerated. It gives a somewhat delusive appearance of brilliancy and clearness which is psychological rather than physical. During the late war a very careful research was made at the instance of the United States Government to determine the actual value of a binocular field glass against a monocular one of exactly the same type, the latter being cheaper, lighter, and in many respects much handier. The difference found in point of actual seeing all sorts of objects under varying conditions of illumination was so small as to be practically negligible. An increase of less than 5 per cent in magnifying power enabled one to see with the monocular instrument everything that could be seen with the binocular, equally well, and it is altogether probable that in the matter of seeing fine detail the difference would be even less than in general use, since it is not altogether easy to get the two sides of a binocular working together efficiently or to keep them so afterwards. There has been, therefore, a definite field for monocular hand telescopes of good quality and moderate power and such are manufactured by some of the best Continental makers. Such instruments have sometimes been shortened by building them on the exact principle of the telephoto lens, which gives a relatively large image with a short camera extension. [Illustration: FIG. 111.—Steinheil Shortened Telescope.] A much shortened telescope, as made by Steinheil for solar photographic purposes, is shown in Fig. 111. This instrument with a total length of about 2 feet and a clear aperture of 2⅜ inches gives a solar image of ½ inch diameter, corresponding to an ordinary glass of more than double that total length. Quite the same principle has been applied to terrestrial telescopes by the same maker, giving again an equivalent focus of about double the length of the whole instrument. This identical principle has often been used in the so-called Barlow lens, a negative lens placed between objective and eyepiece and giving increased magnification with small increase of length; also photographic enlargers of substantially similar function have found considerable use.