The first enables us to see solar systems without number, to which our own, vast as it seems to us, is but as a grain of sand in the desert. The next instrument makes revelations as marvellous of the infinitely minute as does the telescope of the infinitely great, enabling us to see living organizations so small that thirty-two millions could swim in a cubic inch of water. The third, a comparatively modern instrument, reveals the composition of objects, and can detect and register the materials of which the sun and fixed stars are made, or detect an adulteration in wine. It can adapt itself equally to the telescope and microscope, and the very same instrument which will reveal the character of an invisible gas in the Pole-star, when attached to the telescope, can, when connected with the microscope, point out the presence of half a corpuscle of blood where no other instrument could discover any trace of it. All these instruments, together with many others, will be described in the present division of the work, and their analogies with Nature shown. * * * * * WE will now take the subject of the Camera Obscura, an instrument with which the photographic apparatus of the present day has made most of us familiar. As its action depends chiefly upon the glass, or lens, through which the rays of light pass into the instrument, we will first explain that. A “lens” is a glass formed in such a manner that the rays of light which pass through it either converge to a focus, or are dispersed, by means of the law of refraction. Every one who has been photographed--and who has not?--will remember that when the sitter has taken his position, the photographer brings to bear upon him a circular glass fixed into a short tube, and then looks through the instrument as if he were taking aim with some species of firearm. It is no matter of wonder that when savages see the photographic camera for the first time they are horribly frightened, for there is really something weird-like in the appearance of the lens thus presented. Now, this lens is of the shape called “double convex,” both sides being equally rounded, so that a section of it would be shaped very much like a parenthesis (). The effect of this form of lens is to bring the rays of light to a point at a given distance from the centre. This point is called the “focus,” and is well known by means of the common burning-glass, which will set fire to objects placed in its focus, while itself remains quite cool. I have seen lead pour down like water when placed in the focus of a large burning-glass, and even the harder metals will yield to the power of the sun’s rays when thus concentrated.