artificial eye with a focus considerably shorter than his own. He makes use of it by placing his own living eye behind it, and looking through it at that which is beyond it. His own eye is thus brought much closer to the object than it could be without the aid of his lens—the result being an enormous accession of enlarging power. Be it observed, however, that the lens does not augment the size; it only enables the eye to be brought nearer, for could the eye see at a similar short distance without the help of the lens, the result would be precisely the same as that attained by the use of the lens. The real service that our piece of convex glass renders to us is this—it takes the divergent rays which emanate at all points from an object situated nearer to the eye than its natural focus, and brings them into a proper condition to be received by the eye, which is then competent to deal with them in forming a perfect image on its retina. Without the lens this could not be accomplished: the image would then not fall on the retina, but at some distance beyond it.
Let us now apply this principle to the optical part of the compound microscope. At each end of a brass tube some 10 inches in length and 1 inch in diameter are placed the lenses which produce such marvellous results. At one end of the tube nearest to the eye of the observer is placed the eye-piece, at the opposite end is the object-glass, so called from its proximity to the object. We will first deal with the latter: it may be a lens or combination of lenses of high or low magnifying power. By the mechanical aid of rackwork and pinion we contrive to bring the lens into focus with a small object placed on the stage of the microscope. The light transmitted or reflected from this object enters the object-glass and at some distance in the interior of the tube produces a faithful copy of the object greatly increased in size. For the sake of illustration we will consider the copy, or image, thirty times longer, broader, and deeper than the original. Now, although the eye is not present at the spot where this image is formed to receive it, yet we very well know that it is actually there, and placed, too, in a most convenient position to be seen by looking at it through the eye-piece. If the latter had the power of doubling the size of the first image, the result would be a second image twice the size of the first, and sixty times larger in length, breadth, and depth than the object itself. And this is what really takes place in the compound microscope. We have, as it were, two simple microscopes. The first deals with the object, and the second with the enlarged image that object forms. It is in consequence of this compound arrangement that the modern microscope has received its present name.
It is greatly superior to its predecessor, the simple microscope, which labours under the disadvantages of being more limited in its range, the result of producing but one image, and the necessity forced on the observer to have both eye and lens almost in contact with the object he may wish to investigate. The compound microscope, while it preserves us from these drawbacks, secures for us a double advantage. Its magnifying power is much greater in consequence of its ability to produce two images instead of one. And this is secured to us, while the eye remains at the ordinary distance of ten inches from an object.
We have endeavoured to show in a simple manner, without going into optical technicalities, why objects appear so greatly enlarged when seen under the Microscope, and that magnitude, or increase of size, depends on the distance of an object from the eye—that the closer the eye can approach to it and see it, the larger it will appear, and that the lenses employed in the construction of our Microscopes are only helps to bring about such a result.
A DEAD FLY ON THE WINDOW.
Who will prescribe for an unhappy fly? We were led to ask this question in all simplicity the other day, by seeing one of these disturbers of our usual afternoon nap apparently in the agonies of approaching death. As we regarded his rotund and inflated body, vague notions of the veterinary art and even of Mr. Banting flitted through our mind, but help seemed beyond the insectile reach. Its beautiful arrangement of feet to climb glass and walk in an inverted position over our heads, its delicate wings of prismatic hues, its minute spiracles so constructed for breathing as to render a nose a useless appendage, and its numerous eyes which enable it with equal readiness to see a dainty morsel or a sinister enemy, all were about to cease their functions, for disease had arrested its playful gambols and thieving propensities; the sugar basin would "know it no more," and our afternoon nap would be disturbed by one intruder the less.
Still the death of a fly under these circumstances is not without its interest to those
