OF SLEEP. SECT. XVIII. 5, 



obferved that the papillae of the tongue can be feen to become 

 erecled, when we attempt to tafte any thing extremely grattful. 

 Hewfon Exper. knquir. V. ii. 186. Albini Annot. Acad L i. 

 c. 15. Add to this, that the irr mediate organs of fenfe have no 

 objects to excite them in the darknefs and filence of the night ; 

 but their nerves of fenfe nevertheleis continue to poflefs their 

 perfect activity fubfervient to all their numerous fenfitive con- 

 nexions. This vivacity of our nerves of fenfe during the time 

 of fleep is evinced by a circumftance, which almoft every one 

 muft at fome time or other have experienced ; that is, if we 

 fleep in the daylight, and endeavour to fee fome object in our 

 dream, the light is exceedingly painful to our eyes ; and after 

 repeated flruggles we lament in our fleep, that we cannot fee it, 

 In this cafe I apprehend the eyelid is in fome degree opened by 

 the vehemence of our fenfations ; and, the iris being dilated, 

 the optic nerve (hews as great or greater fenfibiiity than in our 

 waking hours. See No. 1 5. of this Seclion. 



When we are forcibly waked at midnight 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 contraction of the iris ; which is owing to the accumulation 

 of fenforial power in the organ of vifion during its flate of lefs 

 activity. But v/hen we have dreamt much of vifible objects, 

 this accumulation of fenforial power in the organ of vifion is 

 leflened or prevented, and we awake in the morning without be- 

 ing dazzled with the light, after the iris has had time to con- 

 tract itfelf. This is 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 accuflomed to, and endeavour to fing it with as little activity 

 of mind as pofTible. Suddenly 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, and then to the other four fides of it ; then 

 get a clear image in your mind's eye of all the fides of the fame 

 cube coloured red ; and then of it coloured green ; and then of 

 it coloured blue ; laftly, open your eyes as in the iormer exper- 

 iment, and after the firft fecond of time allowed for the con- 

 tra&ion of the iris, you will not perceive any increafe of the 

 light of the day, or dazzling , becaufe now there is no accumu- 

 lation of fenforial power in the optic nerve ; that having been 

 expended by its ation in thinking over vifible objects. 



This 



