CORRECTING FOR CHROMATIC ABERRATION.
The amount of correction required is measured by fixing a graduated arc on the base board at the back of the camera, and attaching a small pointer to the handle of the fine adjustment rod (Fig. 734). The object is carefully focussed on the screen, and a plate exposed and developed ; this is placed beside the ground glass screen, and the handle of the fine adjust ment rod is turned until the image on the screen appears as indistinct as it does in the photograph ; then a second plate is exposed. It may be necessary to make two or three exposures before the correct actinic focus is found ; but once this is obtained, the position of the pointer on the scale must be noted, and this serves as a guide in all future exposures. The apparatus devised by Mr. Shipperbottom (Fig. 735) allows six different exposure trials to be made on the same plate, and is useful for saving plates ; it consists of a thin wood screen having a central open ing divided into six segments of a circle ; a circular screen having a diagonal open ing revolves upon this, so that it alter nately exposes and covers each segment.
It is placed in the back of the camera directly in front of the dark slide, and the plate is covered after each exposure while the disc is being moved. By keep ing a record of the position of the pointer on the divided arc when each exposure is made, it is soon found what correction is required to give the best focus. This apparatus may also be used for directly determining the length of exposure necessary for any kind of plate, exposures for different times being made, and the density determined after development.