Word: focals
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...which can be transformed at will from a "photographic" to a "visual" lens by simply turning over the crown-glass lens and changing its distance from the flintglass lens. The Clarks have made for Professor Pickering a lens of this construction with an aperture of thirteen inches and a focal length of fifteen feet...
When the two glasses are finally set in the brass mountings and the lens is complete, the diameter of clear glass inside the fittings will be three feet, the lens having a focal distance of 57 feet! This means that the cylinder of the telescope itself will be at least 57 feet in length, a monstrous "spy glass" indeed...
...this country and in England the objectives and oculars are named in inches or fractions of an inch, relatively to the focal length of a simple lens which will produce the same magnification. Thus taking the focal length of the eye as ten inches, a one inch objective magnifies ten diameters, a 1-50 inch objective, five hundred diameters; a 1-100 objective, one thousand diameters; (the highest powers made). When the ocular system is added, the magnification is equal to the product of the magnifying powers of the objective times the ocular. For instance, a two inch vocular...
...that radiant duad of blueness was looking in my direction. In short, our eyes met; they looked at each other; no - not exactly at each other; that would have been improper. They looked at a sort of middle point lying half-way on the straight line between them, the focal point where their visual rays converged. This of course was very harmless. Pretty soon they smiled, both pairs at the same time; but not at each other, only at the focal point. This certainly was unobjectionable. Unobjectionable? It was more; it was delightful...