Page:Optics.djvu/147

 Sometimes instead of one single strong lens, two or more are used, having together the same power. The advantage gained by this is, that the aberration arising from the spherical form of the glasses is lessened.

As for the rest, whether these lenses be placed close together, or at some distance, they may be equally considered as constituting one single refracting instrument.

160. In this instrument, a double magnification takes place, for an enlarged real image of the object is produced by means of a convex lens, (called an object glass,) and this image is viewed through another lens, the eye glass, in the same manner as an object is with a simple microscope.

In order to find the magnifying power, we will suppose the first image $q$ to be at the least possible distance from the second lens $B$, and we will represent the focal length of the first lens by $F$, that of the second by $F′$, and the distance between them by $b$.

Then the image $q$ is greater than the object $Q$ in the ratio of $Aq:AQ$, that is, of $b−F′+F:F$.

The final image is greater than this would be, when viewed at the least distance of correct vision $1⁄∆″=1⁄∆−1⁄F$, in the ratio of $∆=∆″F⁄∆″+F$, supposing $∆″⁄∆=∆″+F⁄F$ to be as near as possible.

On the whole then, the magnifying power is

The image seen is inverted with respect to the object, as may be seen by the figure.