Report and Transactions of the Glasgow Society of Field Naturalists . eat mistake. The next class of condensers are those for dark ground illumin-atio7i of transjxirent objects, the simplest is the spot-lens. This isa large plano-convex or bi-convex lens, the upper and lower surfacesof which with the exception of a narrow border are ground flatand blackened, so that if the light from the mirror is thrown onit, the marginal rays alone pass through it, and these being muchrefracted pass over the object without entering the object-glass;the result is brilliant illumination of the objects on a bla
Report and Transactions of the Glasgow Society of Field Naturalists . eat mistake. The next class of condensers are those for dark ground illumin-atio7i of transjxirent objects, the simplest is the spot-lens. This isa large plano-convex or bi-convex lens, the upper and lower surfacesof which with the exception of a narrow border are ground flatand blackened, so that if the light from the mirror is thrown onit, the marginal rays alone pass through it, and these being muchrefracted pass over the object without entering the object-glass;the result is brilliant illumination of the objects on a black spot-lens is suitable for object-glasses up to half-inch more perfect instrument for the same purpose is the Paraboloid(Fig. 16) of Mr. Wenham. A parabola is a curve produced bythe section of a cone and of a plane under certain conditions, andit has the property of reflecting all rays, which fall on it parallelto its axis, to one point in the same, called the focus. By rotatingthe parabola round its axis, we get a paraboloid which has the 3/.Y. \ t? N \
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