[Illustration: A TYPICAL REFRACTION ROOM—THE WOOLF SANITARY ALL-METAL EQUIPMENT Installation comprising: Ophthalmic Chair, complete with Ski-optometer, Test Letter Cabinet, Asceptic Trial-Case Cabinet, Muscle Testing and Skioscopic Lamp, Ophthalmometer, Perimeter, Adjustable Tables, Adjustable Stool.] 4. _Comitant and Non-Comitant Deviations_: In some varieties of heterophoria and squint, the amount of deviation is the same in all directions of the gaze, so that the angle between the visual line of one eye and that of the other remains the same, no matter which way the eyes are turned. Such deviations are called comitant or non-comitant, because one eye accompanies and keeps pace with the other in all its movements. In other cases, the deviation changes as the eyes are moved in different directions, so that the angle between the two visual lines constantly varies. Such deviations are termed concomitant. Usually in a non-comitant squint the angle of deviation increases in a regular way as the eyes are moved in one direction and decreases as they move in the direction opposite. In cases of long standing, however, the squinting eye, particularly when very amblyopic, wanders in an uncertain way and apparently quite without reference to the movements of the other eye. CHAPTER XVI LAW OF PROJECTION The movements of the eye are designed primarily to effect fixation—that is, to bring upon the macula the image of the object that we wish to look at. When this has been accomplished, we know as a result of long experience, the direction of the object looked at and also direction of other neighboring objects. This knowledge is doubtless afforded us, in part, by our muscle sense. Thus we know that an object, A, is straight in front of us because we can see it sharply without moving either the head or the eyes from the position of rest or equilibrium; and we know that an object, B, is on the right of us because to see it sharply we have to move either the head or the eyes to the right, thus altering the muscular condition from one of rest to one of tension. But without moving either head or eye, we also know, while still looking at A, that B is to the right, for the image of B is then formed on a portion of the retina situated to the left of the macula. From long experience we also know that an image so situated means an object placed on our right. Moreover, the farther to the left of the macula the image B is, the farther to the right do we judge B itself to be. Similarly, if B is so placed that its image falls below the macula, we then know B itself is really above A, which forms its image on the macula; and if the image of B is above the macula, we know that B itself is below A. The table on page 116 is suggested as a guide in cases of muscular imbalance: SUPPRESSION OF IMAGE