A Manual Of Photography

THE DYNACTINOMETEB. 163 graphometer and dynactinometer, to the vapour of mercury, which developes the two images on each plate. The number of spots given by the photographometer, Fig. 20, will indicate the sensitiveness of the plate ; and in comparing the two images given by the dynactinometer, Fig. 26, 27, accounting for the difference of sensitiveness of each plate, if there is any, we are able at once to determine the comparative power of the two lenses. For the practical investigation of this very important photo- graphic question, Mr. Knight has devised an apparatus that will be most valuable as affording the means of adjusting readily to the best focal distance. Mr. Knight's apparatus consists in a frame having two grooves; one vertical, in which he places the ground glass, and 28. the other forming an angle with the first destined to receive the plate: the planes of the grooves intersect each other in the middle. After having set the focus upon the ground glass, this last is removed, and the plate is placed in the inclined groove. Now, if a newspaper, or any large printed sheet, is put before the camera, the image will be represented on the inclined plate; and it is obvious in its inclination the various points of the plate will meet a different focus. The centre of the plate will coincide with the visual focus ; by its inclination it will in one direction meet the photogenic focus at a point more or less distant from the centre, if the photogenic focus is shorter than the visual focus, and in the other direction if it is longer. The frame is furnished with a scale of division, having the zero in the centre. When the image is represented on the daguerreo- type, by applying against it another moveable scale of division similar to the other, the operator can find what is the division