ABSTRACT
In designing an Experiment the agents and phenomena to be studied are marked off from all others and regarded as the Field of Investigation. All agents and phenomena not included within this field are called Disturbing Agents, and their effects Disturbances; and the experiment must be so arranged that the effects of these disturbing agents on the phenomena to be investigated shall be as small as possible. There are certain primary requisites, however, which are common to all instruments, and which therefore are to be carefully considered in designing or selecting them. The most important kinds of motion with one degree of freedom are, Rotation round an axis; Motion of translation without rotation; and Screw motion, in which a definite rotation about an axis corresponds to a definite motion of translation along that axis.
