



220 O F S L E E P. SECT. XVIII. $. 



nerve (hews as great or greater fenfibility than in 

 our waking hours. See No. 15. of this Sedion. 



When we are forcibly waked at inidmght from; 

 profound fleep, our eyes are much dazzled with 

 the light of the candle for a minute or two, after 

 there has been fufficient time allowed for the con- 

 tra&ion pf the iris ; which is owkig to the accu- 

 mulation of fenforial power in the organ of vifion 

 during its ftate of lefs adivity. But when we have 

 dreamt much of vifible objeds, this accumulation 

 of fenforial power in the organ of vifion is leflened 

 or prevented, and we awake in the morning with- 

 out being dazzled with the light, after the iris 

 has had time to contract jtfelf. This i a matter of 

 great curiofity, and may be thus tried by any one 

 in the day-light. Clofe your eyes, and cover them 

 with your hat ; think for a minute on a tune, which 

 you are accuftomed to, and endeavour to fing it 

 with as little activity of mind as poflible. Sudden- 

 ly uncover and open your eyes, and in one fecond 

 of time the iris will contract itfelf, but you will 

 perceive the day more luminous for feveral feconds, 

 owing to the accumulation of fenforial power in the 

 Optic nerve. 



Then again clofe and cover your eyes, and think 

 intenfely on a cube of ivory two inches diameter, 

 attending firft to the north and fouth fides of it, ancl 

 then to the other four fides of it ; then get a clear 

 image in your mind's eye of all the fides of the 

 iaine cube coloured red ; and then of it coloured 

 green; and then of it coloured blue; laftly, open 

 your eyes as in the former experiment, and after 

 the firft fecond of time allowed for the contraction 

 of the iris, you will not perceive any increafe of the 

 light of the day, or dazzling; becaufe now there is 

 no accumulation of fenforial power in the optic 

 nerve; that having been expended by its adion in 

 thinking over vifiblc obje&s.^ 



