822 Dr. G. J. Stoney on the Limits of Vision : 



several hours, especially under the influence of tea or coffee^ 

 until a feeling of weariness and an indisposition to any pro- 

 longed train of consecutive thought have come over us, I 

 have observed that the revival of visual perceptions, 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 ourselves 

 direct. It is important to note that these visions are rot 

 based on any affection of the retina, and in this respect differ 

 wholly from those spectral images wlrch we see after gazing 

 for some time at objects which somewnat dazzle the sight. 

 These latter shift their position wiuh every movement of our 

 eyeballs ; 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 

 wh'ch 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 myseL" saw appari- 

 tions in this way, after having been three days without sleep, 

 those 1 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 fallen 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 afterwards to another. If we repeatedly 

 seize opportunities of making this experiment, we shall 

 gradually accumulate instances of every degree of vividness, 

 from the full distinctness of a dream in respect of colour, 

 brightness, and form, down to the shadowy dimness of what 

 we very imperfectly see in the exercise of ordinary memory. 

 The same important observation mav be made here as on a 



