It may be well here to mention the effect of some drugs in causing subjective sensations of sight, although these probably depend rather upon the action of the drugs on the brain, than on the eye itself. The centres for sight, according to Ferrier, are the angular gyrus (14 and 15, Fig. 68, p. 185), and the occipital lobes. In delirium tremens arising from alcoholic excess the patients often complain much of visions of the most disagreeable character, which often take the form of demons or of animals.

Cannabis indica produces in some persons, though not in all, visions which may be pleasant or laughable. These chiefly occur just before sleep.1

Salicylate of sodium in some persons tends to cause most disagreeable visions whenever the eyes are shut, and I have seen it have this effect even in such a small dose as five grains. Large doses of digitalis may cause subjective sensations of light, and after taking nearly one grain of digitalin in the course of forty-eight hours I suffered from the centre of the field of vision being occupied by a bright spot surrounded by rainbow colours. Digitalin when introduced into the eye locally causes at first smarting and lacrimation, which soon passes off, but after four or five hours, when a light is looked at, a halo is seen surrounding it, which is not improbably due to some opalescence in the cornea.2