This lens really consists of two achromatic combinations, the pair at the front being cemented together, and that at the rear having an air space between. The combination is so designed that the spherical aberration of the one pair neutralises that of the other, and the result is or should be a lens corrected both for chromatic and spherical aberration.
These lenses, however, vary very much in the perfection of their results, and as they are at present usually imported in bulk from France, the customer does well to insist upon a demonstration of his own particular lantern before acceptance.
The magnifying power of a lens depends upon its 'focus' multiplied by its distance from the screen, and the focus in the case of a simple lens is easily determined by the familiar 'burning-glass' experiment, that is by focussing an image of the sun upon a piece of paper and measuring accurately
the distance the lens must be away to produce the most concentrated spot.
In practice it is sufficiently accurate to focus a distant window, or other luminous object, upon the paper, any error obtained by this method being for ordinary purposes a negligible one.
With a compound lens, such as a 'Petzval' combination, this method does not hold good, as the optical centre of such a lens is not necessarily midway between its two components.
The actual focus can be got pretty approximately by focussing a window or other object as before and measuring the distance from one definite point (say the front edge of one of the lens cells) to the paper, then turning it round and taking a second measurement from the same point, the mean between the two measurements giving the actual focus.
In practice the 'simple equivalent focus,' as it is termed, of a lantern lens is usually determined by measuring the magnification of the image thrown upon the screen, when, by knowing the original size of the slide (a 'standard' slide of 3 inches diameter is usually taken) and the distance between lantern and screen, we get the focus from the following very simple equation:
Diameter of picture on screen (in feet) / Diameter of slide (in inches) = Distance between lens and screen (in feet) / Focus of lens (in inches)
or perhaps more simply still: