Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration / by Lewis Wright.

  • Wright, Lewis, 1838-1905.
Date:
1901
Catalogue details

Licence: Public Domain Mark

Credit: Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration / by Lewis Wright. Source: Wellcome Collection.

    17/488 (page 5)
    A convex or focussing lens must act in the same way, because it is a piece (or combination of pieces) of glass formed so as to be thickest in the middle, the faces gradually becoming more and more inclined till they go off to an edge. It is a circular prism, of gradually-increasing inclination towards its edges. If we place such a lens in a beam of parallel rays, as from the sun, it is easy to see what must happen. The centre ray, striking the glass perpendicularly, proceeds straight on unrefracted. The next outer rays, meeting the glass at a small angle, are a little bent in towards the centre or thickest part. Then rays farther out from the centre are more bent in, because they strike the surfaces at a greater angle ; and so the whole beam of rays meet practically in one point, f (fig. 3), which is the ‘ focus ’ for parallel rays, or ‘ principal focus.’ This is the well-known phenomenon of a burning-glass. Take the converse, however ; if the rays diverge from the luminous point f at this focus of the lens, they are refracted just the same, being converted into parallel rays. Suppose, however, that the rays diverge from a point, /, farther away from the lens than its principal focus, f (fig. 4). What must happen then is, obviously, that all the rays diverging from the point / will be bent in so as to meet again in some other point, f2, on the other side of the lens, which point must have