_Exper._ 11. In the Sun's Light let into my darken'd Chamber through a small round Hole in my Window-shut, at about ten or twelve Feet from the Window, I placed a Lens, by which the Image of the Hole might be distinctly cast upon a Sheet of white Paper, placed at the distance of six, eight, ten, or twelve Feet from the Lens. For, according to the difference of the Lenses I used various distances, which I think not worth the while to describe. Then immediately after the Lens I placed a Prism, by which the trajected Light might be refracted either upwards or sideways, and thereby the round Image, which the Lens alone did cast upon the Paper might be drawn out into a long one with Parallel Sides, as in the third Experiment. This oblong Image I let fall upon another Paper at about the same distance from the Prism as before, moving the Paper either towards the Prism or from it, until I found the just distance where the Rectilinear Sides of the Image became most distinct. For in this Case, the Circular Images of the Hole, which compose that Image after the same manner that the Circles _ag_, _bh_, _ci_, &c. do the Figure _pt_ [in _Fig._ 23.] were terminated most distinctly without any Penumbra, and therefore extended into one another the least that they could, and by consequence the Mixture of the heterogeneous Rays was now the least of all. By this means I used to form an oblong Image (such as is _pt_) [in _Fig._ 23, and 24.] of Circular Images of the Hole, (such as are _ag_, _bh_, _ci_, &c.) and by using a greater or less Hole in the Window-shut, I made the Circular Images _ag_, _bh_, _ci_, &c. of which it was formed, to become greater or less at pleasure, and thereby the Mixture of the Rays in the Image _pt_ to be as much, or as little as I desired. [Illustration: FIG. 24.]