To assist us in gaining a clearer notion to the mode in which a single lens serve to magnify minute objects, it is necessary to take a passing glance at the ordinary phenomena of vision. The human eye is so constituted, that it can only have distinct vision when the rays falling upon it care parallel or slightly divergent; because the retina, on which the image impinges, requires the intervention of the crystalline lens to bring the rays to an accurate focus upon its surface.

The limit of distinct vision is generally estimated at from six to ten inches; objects viewed nearer, to most persons, become indistinct, although they may be larger. The apparent size of an object is, indeed, the angle it subtends to the eye, or the angle formed by two lines drawn from the centre of the eye to the extremity of the object.

Now the utility of a convex lens interposed between a near object and the eye consist in its reducing the divergence of the rays forming the several pencils issuing from it, so that they enter the eye in a state of moderate divergence, as if they had issued from an object beyond the nearest limit of distinct vision; and a well-defined image is consequently formed upon the retina. A double-convex lens is placed before the eye, near is a small arrow , to represent the object under examination ; and the cones drawn from it are portions of the rays of light diverging from those points and falling upon the lens.

These rays, if permitted to fall at once the pupil, wood be too divergent to allow of their being brought to a focus upon the retina by the dioptric media of the eye. But being first passed through the lens, they are bent into nearly parallel lines, or into lines diverging from some points within the limits of distinct vision. Thus altered, the eye receives them precisely as if they had emanated directly from a larger arrow placed at ten inches from the eye. The difference between the real and the imaginary arrow is called the magnifying power of the lens.

The objects, when thus seen, appears to be magnified nearly in the proportion which the focal distance of the lens bears to the distance of the object when viewed by the unassisted eye; and is entirely owing to the object being distinctly viewed so much nearer to the eye than it could be without the lens. With these preliminary remarks as to the medium by witch microscopic power is obtained , we shall proceed to apply them to the construction of perfect instrument.