554 



Mr. P. L. Gray on the Minimum 



Observations on two succeeding days gave readings varying 

 from 470° in the morning to 385° in the evening, the influ- 

 ence of rest in darkness being always plainly marked. 



To test if the intervals of rest noted above were sufficient 

 to bring the eyes to their extreme state of sensitiveness, some 

 observations were made at 3 a.m., after I had been asleep in 

 a dark room for 3 hours. The readings then obtained were 

 373°, 386°, showing a slightly greater sensitiveness than that 

 in any previous experiment ; the small difference probably 

 indicates that the eyes were very near their extreme limit, 

 and that no longer rest would give a lower reading. 



Before going on to the general conclusions to be drawn 

 from these results, the values are given obtained by other 

 observers, who (with the exception of two) were either mem- 

 bers of the staff, or students, of Mason College. They are 

 probably not equally trustworthy, and in general no particular 

 preparation was gone through by the observer. 



Usually the first observation in each case showed a higher 

 temperature than the second and succeeding ones. This was 

 to be expected, since the time spent in darkness during the 

 first experiment prepares the eye to a certain extent for the 

 second. The results obtained are as follows : — 



Case A. 



Mean temperature 



. . 432° C. 



„ B. 



?5 ?> 



. . 422 



„ C. 



)) V 



. 436 



„ D. 



)> V 



. . 409 



,, -u. 



11 J5 



. 428 



» *'• 



» v) 



. 438 



„ I- 



V » 



. . 426 



„ J. 



?? •)■) 



. . 440 



All the above were taken either in the morning or early in 

 the afternoon. 



Case H. Mean temperature . . 408°. 



In this case the room was darkish. 



Case G. Evening ..... 388° 



Morning 435° 



Evening 380°-385° 



The last observation was made after 3 7 minutes had been 

 spent by the observer with his eyes shut in the nearly dark 

 laboratory. 



