Related to Ernst Mach (trans. C.M. Williams), Contributions to the Analysis of the Sensations (Chicago, 1897 [German ed. Prague 1885]).
Description:

'2.

... If I look straight before me, fixing my eyes upon an object O, an object A, which is reflected on the retina in a, at a certain distance below the point of most distinct vision, appears to me to be situated at a certain height. If I now raise my eyes, fixing them upon B, A retains its former height. It would necessarily appear lower down if the position of the image on the retina, or the arc oa, alone determined the space-sensation. I can raise my glance as far as A and farther without a change in this relation. Thus, the physiological process which conditions the voluntary raising of the eye, can entirely or partly take the place of the height-sensation, is homogeneous with it, or, in brief, algebraically summationable with it. If I turn my eyeball upward by a slight pressure of the finger, the object A actually appears to sink, proportionately to the shortening of the arc oa. The same thing happens when, by any other unconscious or involuntary process — for example, through a cramp of the muscles of the eye — the eyeball is turned upward. According to an experience now familiar to opticians for some decades patients with paralysis of the rectus externus reach too far to the right in attempting to grasp objects at the right. Since they need to exert a stronger impulse of the will than persons of sound eyes, In order to fix their glance upon an object to the right, the thought naturally suggests itself that the will to look to the right determines the optical space-sensation "right." Some years ago, i I put this observation into the form of an experiment, which everyone can try for himself. Let the eyes be turned as far as possible towards the left and two large lumps of moderately hard putty firmly pressed against the right side of each eye-ball. If, now, we attempt to glance quickly to the right, we shall succeed only very imperfectly, owing to the incompletely spherical form of the eyes, and the objects will suffer a strong displacement to the right. Thus the mere will to look to the right imparts to the images at certain points of the retina a larger "rightward value," as we may term it for brevity. The experiment is, at first, surprising. It will soon be perceived, however, that both facts —viz., that by voluntarily turning the eyes to the right, objects are not displaced, and that by the forced, involuntary turning of the eyes to the right, objects are displaced to the left — together amount to the same thing. My eye, which I wish to, and cannot, turn to the right, may be regarded as voluntarily turned to the right and compulsorily turned back by an outer force.

3.

The will to perform movements of the eyes, or the innervation to the act, is itself the space-sensation. This follows naturally from the preceding consideration. If we have a sensation of itching or pricking in a certain spot, by which our attention is sufficiently secured, we immediately grasp at the spot with the correct amount of movement. In the same manner we turn our eyes with the correct amount of exertion towards an object reflected on the retina, as soon as this exerts a sufficient stimulus to draw our attention. By virtue of organic apparatus and long exercise we hit immediately upon the exact degree of innervation necessary to enable us to fix our eyes upon an object reflected on a certain point of the retina. If the eyes are already turned towards the right, and we begin to give our attention to an object further to the right or the left, a new innervation of the same sort is algebraically added to that already present. A disturbance of the process arises only when extraneous, involuntary innervations or outward moving forces are added to the innervations determined by the will.' (58-60)