i88 THE POPULAR SCIENCE MONTHLY, 



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 tele- 

 scopic 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. 



THOMASYILLE AS A WINTER EESORT. 



By E. L. YOUMANS. 



AS the winter season approaches in the Northern States and in 

 Canada, wdth 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 aug- 

 menting, 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 



