This measurement was always made in the morning. The turning of the eye to a new point of regard in response to the sudden appearance or movement of some object which is removed from the line of clearest vision is an important reaction for the organism. It would seem that, considering the multitude of reactions of this character which an individual makes in the course of his everyday life, an adult subject in coming to the Laboratory would be perfectly practiced in this regard. In any wide range of neuro-muscular measurements, particularly when new and unpracticed subjects are to be used, this measurement of eye reactions is an especially promising one for consideration and use. Suitable apparatus for photographically recording the eye reactions has been designed and used by Dodge and has been fully described by him.2 Briefly, the record is made by photographing a beam of light reflected from the cornea of the subject's eye. This calls for a source of illumination, an adjustable head-rest, mirrors, a lens, a camera of suitable design to move the photographic plate, and some device for presenting stimuli to the eye.

Several modifications and additions have been made to the apparatus as used by Dodge and Benedict in the alcohol investigation. Certain definite aims were in mind in making these changes: (1) a form of stimulus device was desirable, which would provide more variety of stimulus positions and still give no secondary criteria to the subject; (2) the field of view of the subject should include only what was necessary for him to see; (3) it should be possible to place a new subject in position quickly and to inform him definitely of his task; the subject who is familiar with the test should be able to place himself in position, adjusting the headrest as directed by the operator; (4) for laboratory economy, as well as to avoid the possibility of giving secondary criteria to the subject, the apparatus should be operated by one person, and from one position; (5) more reactions should be taken to get a better sample of the subject's performance. These points were met, as described in the following paragraphs.

1 The records for some readers might be more easily understood in connection with the actual movements of the muscle if the figure is turned upside down; a rise in the curve will then be seen to correspond to a rise or a thickening of the muscle. 2 Dodge, An experimenta study of visual fixation, Monograph Supplement of the Psychological Review, No. 35, 1907. See also Dodge and Benedict, Carnegie Inst. Wash. Pub. No. 232, 1915, p. 79.

As Dodge has pointed out, there is no limit to the number of reactions that can be recorded on the same photographic plate, except that beyond a certain number they will become confused with each other and, on that account, illegible. It appeared to us that reactions, photographed on the plate at intervals of one-eighth inch, would be sufficiently legible, provided the subject kept his head quiet in the head-rest. Accordingly a device was arranged to move the Dodge falling-plate camera laterally by small increments. See V, W, and T in figure 30, also V in figure 41. A treadle T, shown in the lower left-hand corner of figure 30 and worked by the operator's left foot, was kept in position by a spring and connected to a pawl, which, when drawn down, engaged with a toothed wheel at V. From the shaft on which the toothed wheel was mounted a light cord connected with the body of the camera. When the wheel was turned to the left, therefore, the camera was drawn to the left; the weight W kept the string taut and made the movements to the left fairly easy. The camera was supported on two slides which moved easily on the surface of the table. Usually the camera was moved over one step for each reaction, unless it was necessary to focus between reactions, when it was often moved two steps. By this method of moving the camera it is not necessary to focus so frequently as when the camera is moved by hand, which is usually a rougher judgment. The device made it practical to take 15 or 16 reactions on one plate 2 1/2 by 7 inches in size. The camera could be moved freely right and left when the operator's foot was not on the treadle, T.

In beginning to take records on a plate the camera was moved to the right, and the beam of fight from the subject's eye focused on the glass G, below the plate P, and at a position indicated by the small cross. (In the Dodge camera the arrangement which holds the plate-holder containing the photographic plate, P, moves downward with the flow of oil through the by-pass cylinders, B.1) The up and down movements of the plate-holder were made to open and close a shutter, S, by means of the cords 1 and 2. When the plate-holder was drawn up by hand, just before it reached its highest position, and when the ground glass, G, came opposite the shutter S, string 1 became taut and opened the shutter. This remained open until the fall of the plate was complete. String 2 then became taut and closed the shutter. Thus the plate was protected from fogging, except during the actual moments when reactions were being recorded or the camera was being focused.