LENS. In Optics, a thin piece of glass or any other transparent substance, bounded on both sides by polished spher ical surfaces, or on the one side by spherical and on the other by a plain surface ; and having this property, that parallel rays of light, in passing through it, have their direction changed, so as to converge to a given point, called the principal focus of the lens, or to diverge as if they proceeded from that point.
Lenses are, in fact, mere multiplying glasses, with an infinite number of sides, producing an infinite number of images, whose visual resultant is one blended fig ure, expanded over the whole visual angle of the glass, in length and in breadth, and therefore said to be magnified. The images produced by the inclined or ob lique sides, owing to unequal refractions, are however highly-colored in the focus ; and, owing to the unequal inclinations of the spherical form, the rays do not all converge exactly to the same point. Len ses have therefore been very properly com posed of two kinds of glass, which re fract differently or unequally, and then, by combining a convex and a concave, the inequality is destroyed, and the im age free from color. The forms too have been varied from the spherical to the parabolic, with a view to concentrate the rays in one point. Lenses are manufac tured with great recision, by steam power, by Jenkins 's machine, fixed in concave basins, and the friction proceeds on some hundreds at the same time.
A spherical lens, shown at A, is a sphere or globule of glass.
Lenses receive different denominations, according to their different forms.
A double convex lens, shown at B, is a solid formed by two Convex spherical sur faces ; and is equally convex or unequally convex, according as the radii of its two surfaces are equal or une qual.
A piano-convex lens, C, is that of which one of the surfaces is plane and the other convex.
A double concave lens, I), is bounded by two concave spherical surfaces, which have either the same or a different cur vature.
A plano-con cave lens, E, has one stir face plane, and the other con cave.
A meniscus, F (so called from its re semblance to a little moon,) is a lens of which one of the surfaces is convex and the Other concave, and which meet if con tinued. The radius of the convex sur face is consequently smaller than the radius of the concavo.
A concavo-convex lens, G, is that of which one of the surfaces is concave and the other convex ; but in this case the surfaces will not meet though continued, the radius of the concave surface being smaller than that of the convex one.
The straight line M N which passes through the centres of all the curved sur faces, or is perpendicular to both surfaces of the same lens, is called the axis of the lens ; and it is in this line that the focus of the lens is situated.
It was observed, at an early period, that a transparent body of a spherical form has the property of collecting at the focus the parallel rays of light which fall on its surface. But it was remarked, at the same time, that the illumination at these foci was extremely feeble, in consequence of the thickness of the glass which the light had to pass through. This incon venience is removed by taking only two small segments instead of the entire sphere ; by which means, as the refrac tion takes place only at the surfaces, and not in the interior of the glass, the very same refraction of the rays is produced as when the whole sphere is used ; and the thickness of the glass being greatly diminished, the rays pass through it in much greater number, and the intensity of the light in the focus is much more considerable.
The rules for finding the focal distances of the different sorts of lenses are the following. They depend in some meas ure on the refracting power of the glass. We shall here suppose the index of re fraction to be 1.500.
Lenses of great power and correct form are made in this country.