[1] Muensterberg, H., and Campbell, W.W.:  PSYCHOLOGICAL REVIEW,
   I., 1894, p. 441.

The origin of such compensatory reactions is connected with the permanent relations of the whole bodily organism to the important objects which surround it.  The relations of the body to the landscape are fairly fixed.  The objects which it is important to watch lie in a belt which is roughly on a horizontal plane with the observing eye.  They move or are moved about over the surface of the ground and do not undergo any large vertical displacement.  It is of high importance, therefore, that the eye should be capable of continuous observation of such objects through facile response to the stimulus of their visual appearance and movements, in independence of the orientation of the head.  There are no such determinate spatial relations between body position and the world of important visual objects in the case of those animals which are immersed in a free medium; and in the organization of the fish and the bird, therefore, one should not expect the development of such free sensory reflexes of the eye in independence of head movements as we know to be characteristic of the higher land vertebrates.  In both of the former types the eye is fixed in its socket, movements of the whole head or body becoming the mechanism of adjustment to new objects of observation.  In the adjustment of the human eye the reflex determination through sensory stimuli is so facile as to counteract all ordinary movements of the head, the gaze remaining fixed upon the object through a series of minute and rapidly repeated sensory reflexes.  When the eyes are closed and no such visual stimuli are presented, similar reflexes take place in response to the movements of the head, mediated possibly by sensations connected with changes in position of the planes of the semicircular canals.

VII.

If eye-strain be a significant element in the process of determining the subjective horizon, the induction of a new center of muscular equilibrium by training the eyes to become accustomed to unusual positions should result in the appearance of characteristic errors of displacement.  In the case of two observers, A and H, the eyes were sharply raised or lowered for eight seconds before giving judgment as to the position of the illuminated spot, which was exposed at the moment when the eyes were brought back to the primary position.  The effect of any such vertical rotation is to stretch the antagonistic set of muscles.  It follows that when the eye is rotated in the contrary direction the condition of equilibrium appears sooner than in normal vision.  In the case of both observers the subjective horizon was located higher when judgment was made after keeping the eyes raised, and lower when the line of sight had been depressed.  In the case of only one observer was a quantitative estimation of the error made, as follows:  With preliminary raising of the eyes the location was +36’.4; with preliminary lowering, -11’.4.