difficult to calculate the exposure for any distance or any more or less actinic light. For example, the exposure required for the same negative at a distance of 36 ins. from the same gas burner is easily calculated by the rule that the exposure alters as the square of the distance between the light and sensitive paper. The exposure required at 18 ins. 8 sees.is the ex-posure required at 36 ins. will be in the ratio of, or as
if the exposure in the first case = 8 sees., the exposure in the second case = 8x4 =32 sees. This will explain the use of the actinometer, and deter-mine the first factor. (For other actinometers see Exposure.)
(2) The Density of the Negative. It is difficult to accu-rately determine this, as the actual deposit of silver does not alone represent the density of the negative. The colour of the deposit, and the presence or absence of stain in the film, will also influence this factor; but by using the actino-meter as suggested under the first factor we practically determine the second factor also.
(3) The Intensity Ratio of the Stop. Most workers know the usual definition of this term, which is the ratio the aperture of the stop or diaphragm bears to the equivalent focus of the lens; when using a lens for enlarging, however, we never use it at its equivalent focus, the focus altering with the degree of enlargement. Therefore we have to calcu-late anew the intensity ratio of our stops for the new focus. Thus, supposing we are using an focus lens for an enlargement of a quarter-plate to 12 by 12, or, in other words, if we are enlarging four times, the focus of our lens becomes in.; therefore all the diaphragms will be pro-portionately reduced in ratio diameter. It will always be found more convenient if diaphragms of definite diameters are used. Thus special diaphragms of 1-in. diameter aperture or diameter aperture can be obtained; and it is thus easy to cal-culate at once the new intensity ratio, without troubling to measure the diameter every time. (See Diaphragms.)
(4) The Number of Times of Enlargement, or the Distance between the Lens and Sensitive Surface. It is requisite to take into account this factor, because, according to the well-known rule, the intensity of illumination coming from a point and impinging on a given surface is inversely ms the square of its