234 Scientific Proceedings, Royal Dublin Society. 
some circumstances, this is distinctly the case. When, unfortu- 
nately, we lie awake for several hours, especially under the 
influence of tea or coffee, until a feeling of weariness and an 
indisposition to any prolonged train of consecutive thought have 
come over us, I have observed that the revival of visual per- 
ceptions, when thinking about past scenes, becomes stronger 
and is easily perceived, and that in some cases it may become 
almost vivid. In extreme cases it even amounts to a kind of 
dreaming with the eyes open—the dream, however, differing from 
ordinary dreams by being one the progress of which we can our- 
selves direct. It is important to note that these visions are not 
based on any affection of the retina, and in this respect differ 
wholly from those spectral images which we see after gazing for 
some time at objects which somewhat dazzle the sight. These 
latter shift their position with every movement of our eye-balls; 
the others retain what we estimate to be their positions in space, 
notwithstanding that the eyes be moved about. Now this is very 
significant. It shows that the train of physical causes which lead 
up to that event in the posterior lobe, which is the adjunct of our 
perception of these visions, did not originate in the retina, but in | 
a part of the brain where it could arise in conjunction with some 
of those events which are the physical adjuncts of our judgments 
about space. This is an important conclusion to have reached. 
What is probably in reality only a further stage of these 
waking dreams is sometimes experienced in fever, when the patient 
has been for days without sleep. I myself saw apparitions in this 
way, after having been three days without sleep, those I saw 
having a marvellous appearance of reality, and being seen in the 
daylight when I could at the same time see in the ordinary way 
the objects about me in the room, except where one of these novel 
figures intruded. In these places the connexion with the retina 
seems to have been rendered more or less inoperative, and a visual 
perception, otherwise produced, was substituted for the ordinary 
one. 
Another instructive and more agreeable way of making the 
observation is to experiment on ourselves when in that stage of 
drowsiness in which we seem to have gone partially asleep, but 
not so much so but that we can still voluntarily direct our thoughts 
to some well-remembered scene, or still better, first to one, and 
