Temperature of Visibility. 553 



The first case given is my own, in which it has naturally 

 been easier to obtain a larger number of records than in the 

 case of other people. 



The dates are given merely to distinguish one day's obser- 

 vations from another's. 



The letter B after any temperature signifies that the surface 

 was lampblacked in that experiment ; otherwise it was bare, 

 and in its ordinary condition of polish. 



The details of the observations are as follows : — 



Feb. 10th. 451°, 460°, 460°, 444°, 447°, 448°. Observations 

 made in the morning, without any special pre- 

 paration as to resting the eyes. 



„ 14th. 427°, 422°, 422°. Evening. 



„ 15th. 437°(B),432 (B),427 (B),427 ,427°. Evening. 



„ 16th. 422°, 418°, 421°, 419° (B), 419° (B). Evening ; 

 2nd observation after two minutes in complete 

 darkness. 



„ 19th. 410°, 410°. Evening. 



„ 22nd. 409°, 409°, 408°. Evening. 



After these observations more attention was paid to the 

 time of day and the state of preparation of the eyes. 



Feb. 26th. 401°, 397°; 388°, 394°, 384°; 398°, 403°. Even- 

 ing. Between 2nd and 3rd observations, 11 

 minutes were passed in perfect darkness; after 

 the 5th, 10 minutes in writing under a bright 

 incandescent lamp. 



Before the next set, 16 minutes with eyes 

 shut in nearly dark room : — 

 383°, 392°, 392° (B), 409° (B), 409° (B), 416°, 41 7°, 

 409°. 



„ 27th. 470°, 464°, 461°, 463°. Morning (bright). 



413°, 414° ; 392°, 400°, 400° ; 410°. Evening ; 

 3rd observation after half an hour with eyes 

 shut. Several more observations were made, 

 showing a regular fall of temperature after 

 resting the eyes in the dark, and a rise after 

 reading &c. the limits being about 390° and 410°. 



„ 28th. 455°, 447°, 447°; 459°. Morning (dull). 



„ „ 453°: 417°, 415°, 419°. Afternoon; 4 minutes 

 in darkness after 1st observation. 



„ „ 407°; 389°, 389°; 404°, 399°; 392, 392°, 393°. 

 16 minutes in darkness between 1st and 2nd 

 observations ; 12 minutes' reading between 3rd 

 and 4th, and again a few minutes' rest after 

 the 5th. 



