This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Note. It must be understood that these rules regarding the direction of the eye pertain to Plain Portrait Lighting, and do not apply to extreme or odd positions and odd lightings or Genre work. In the latter case be guided entirely by the effects desired.
Eye Rest. Some point on which to rest the eyes should always be suggested to the subject. The best eye-rest is the face of the photographer. A clever photographer can change the expression of his subjects instantly, by having them rest their eyes on him. He can better guide the eye for height and angle by rising or stooping. He can also have the eye lead the face at any angle he desires, by simply moving about more or less in one direction or the other.
146. Another advantage of the human eye-rest is, that at the proper time a clever remark from the photographer will bring an expression that otherwise would be impossible to obtain. It is advisable to practice talking to your subject continually while preparing for the exposure. This may seem difficult at first, but it will come easy by practice, and you will find it a great advantage in obtaining expression. There are some subjects who cannot look at the photographer during an exposure. In such cases you should hold one hand in the direction you wish the subject to look and have the eyes follow the hand. Moving the hand about slightly will overcome any stare that might occur if the subject were to look steadily at one particular spot. It is also permissible to have them look toward some space on the wall, but never give them a small, defined space to look at, because the larger the space the easier the eye will rest. The latter method, however, is not recommended as it seldom produces the required expression. The subject is apt to appear a trifle stiff and the expression is more than likely to be staring and unnatural.
147. Another point should be taken into consideration, and that is, that the focal length of the eyes of all subjects not the same. For instance, some subjects in looking at you from a normal distance expand or contract the pupil of their eyes, and often assume a staring appearance. You should then move about toward or from your subject until you observe that their eyes are properly focused. This will not occur frequently, as usually the distance between the camera and the subject is the normal distance upon which all eyes may rest easily. However, in extreme cases, for near or far sighted subjects you will need to apply some method similar to those just mentioned.
148. Never tell the subject where to look until you have drawn the slide and are ready to make the exposure, and then, without any further caution, attract their attention to your face by talking to them, making some remark that will bring forth the proper expression; but, in case the eyes of the subject cannot rest easily when looking at you, have them look at some distant point and instantly make your exposure, before the eyes have had a chance to become set.