794. It is convenient to have a practical standard for unity. An image of the same size as the original is a familiar case, and serves as such standard. By dividing the figures in the third column by four, we get at the figures in the last column, which represent the exposure required for varying degrees of enlargement or reduction, compared with the exposure for a copy of the same size.

795. The table is carried up to enlargements of thirty diameters. That is about the amount required for enlarging a carte-de-visite to life size.

796. The exposures required in reductions do not vary to the same extent that they do in enlargements. It has. therefore, not been thought necessary to fill in the steps between images of 1-10 and 1-20, and between 1-20 and 1-30 of the size of the original. Beyond 1-30 there is scarcely no perceptible difference in the exposure until disturbance comes from another cause, a considerable distance of illuminated atmosphere (haze or fog) intervening.

797. The figures in the second column will also serve as a table for distances from the lens to the plate, and to the original; all that is necessary being to multiply by the principal focus of the lens in use. In the case of enlargements the figures less than 2 must be multiplied to get the distance from the original to the lens, and the figures greater than 2 for the distance from the lens to image. For reductions, the figures less than 2, multiplied by the principal focus of the lens, yield the distance from lens to plate; and the figures higher than 2, similarly multiplied, give the distance of original from lens.

798.

With a double lens it is usually sufficient to measure from the position of the diaphragm-plate.