THE BRAZEN SERPENT. 135 serpent of brass elevated in the midst of the camp,and looking, lived. It addresses the other, that, “beholding as with open face in a glass the glory of the Lord, they may be changed into the same image from glory to glory, as by the Spirit of the Lord." 2 Cor. iii. 18. May this be the privilege of every one who reads these pages! CHAPTER XI. LENSES-THE TELESCOPE THE MICROSCOPE-VARIOUS OPTICAL INSTRUMENTS. To introduce the interesting facts which remain to be stated, it must be premised, that the five rays of light in the following figure-the representatives indeed of innumerable rays-issuing from the point c, are said to form a pencil of light. It is also evident that to make all such rays converge or meet again in one place, as ƒ, beyond a transparent body placed at a b, it would be necessary, while the middle ray cd remained straight, for the others to be bent more and more, in proportion to their distance from the centre d. According, then, to the law of refraction, that light entering from air through the surface of any denser medium, is bent there towards the perpendicular,-if a piece of glass were placed at a b, of such a form that the rays falling on it from c should meet and leave its surfaces with greater and greater obliquity, in proportion as the points of incidence are more distant from the centre, the purpose Now a glass having a regular convexity will answer this purpose; it is therefore represented, with its effect, in the figure; the ray in the centre going straight through to f, and the others being bent in proportion to their distance from the centre. The point ƒ is generally called the focus, of which an explanation has already been given. The focus, it should be observed, is always in the direction of the central ray of the pencil of light: if the light issue from o, it will be at p; and if it come from x, it will be at z. This property of a lens to bring together all the rays of light which traverse it from any one part of an object placed before it, renders it of great value. If, |