Single microscopes magnify the diameter of the object,[26] as we have already shewn, in the proportion of the focal distance (to the limits of distinct vision with the naked eye) to eight inches. For example, if the semi-diameter of a lens, equally convex on both sides, be half an inch, which is also equal to its focal distance, we shall have as ¹⁄₂ is to 8, so is 1 to 16; that is, the diameter of the object in the proportion of sixteen to one. 2. As the distance of eight inches is always the same, it follows, that by how much the focal distance is smaller, there will be a greater difference between it and the eight inches; and consequently, the diameter of the object will be so much the more magnified, in proportion as the lenses are segments of smaller spheres. 3. If the object be placed in the focus of a glass globule or sphere, and the eye be behind it in the focus, the object will be seen distinct in an erect situation, and magnified as to its diameter, in the proportion of ³⁄₄ of the diameter of the globule to eight inches; thus suppose the diameter of the sphere to be ¹⁄₁₀ of an inch, then ³⁄₄ of this will be equal to ³⁄₄₀; consequently, the real diameter of the object to the apparent one, as ³⁄₄₀ to 8, or as 3 to 320, or as 1 to 106 nearly. [26] Cyclopedia, Article Microscope. OF THE DOUBLE OR COMPOUND MICROSCOPE. In the compound microscope, the image is viewed instead of the object, which image is magnified by a single lens, as the object is in a single microscope. It consists of an object lens N L, Fig. 5. Plate I. and an eye glass F G. The object B O is placed a little further from the lens than its principal focal distance, so that the pencils of rays proceeding from the different points of the object through the lens, may converge to their respective foci, and form an inverted image of the object at Q P; which image is viewed by the eye through the eye glass F G, which is so placed, that the image may be in its focus on one side, and the eye at the same distance on the other. The rays of each pencil will be parallel, after passing out of the glass, till they reach the eye at E, where they will begin to converge by the refractive powers of the humours; and after having crossed each other in the pupil, and passed through the crystalline and vitreous humours, they will be collected in points on the retina, and form a large inverted image thereon. It will be easy, from what has been already explained, to understand the reason of the magnifying power of a compound microscope. The object is magnified upon two accounts; first, because if we viewed the image with the naked eye, it would appear as much larger than the object, as the image is really larger than it, or as the distance f R is greater than the distance f b; and secondly, because this picture is again magnified by the eye glass, upon the principle explained in the foregoing article on vision, by single microscopes.