Page:Popular Science Monthly Volume 28.djvu/198

188 four times the common radius of curvature of the three surfaces just mentioned.

The second is that derived by Dr. Charles S. Hastings from an elaborate mathematical investigation of every possible form of telescopic object-glass. In this form, on the contrary, the concave flint-glass lens is placed in front of the convex crown-glass lens, and close to it. The two inner surfaces have nearly the same curvature; the two outer surfaces, though not quite alike, have a curvature whose radius differs but little from three and a half times that of the inner surfaces. The focal length of this object-glass is about four times the radius of curvature of the inner surfaces. This form of object-glass gives the sharpest definition attainable with the use of only two kinds of glass whose surfaces are of reasonably small curvature.



s the winter season approaches in the Northern States and in Canada, with its dangers to many and its discomforts to all, the question will be often asked, "Where shall we go to secure the best advantages of a milder climate?" The obvious, and with many the sufficient, answer will be, "Go South, where it is warmer." This may be satisfactory for the numerous and increasing class of well-to-do, leisurely, and healthy people who seek a change of climate purely as a matter of personal enjoyment. They are simply in quest of pleasurable sensation, and their instincts may be trusted to find the nicest places with luxurious accommodations, ample amusements, social gayety, and whatever can make the time pass pleasantly; and when they get tired of one place they can find another with fresh novelties and attractions. But, wherever they go, these people are extremely useful. They constitute the great mass of the patrons of Southern winter resorts. Their numbers each year are rapidly augmenting, and the money they spend contributes materially to promote those increasing facilities of travel, hotel-accommodations, and town-improvements of which all share the advantage.

But there are a good many others to whom the question, where to go to escape the inclemencies of a Northern winter, is less simple and more serious. These are invalids laboring chiefly under various forms of pulmonary trouble. When such are advised by the physician to seek a more congenial climate, the question where to go becomes urgent and often perplexing. Happy the patient advised to change his climate when the physician knows enough to give him intelligent instructions as to whither he shall proceed. Does he need a mild or a 