170 ANNUAL OF SCIENTIFIC DISCOVERY. 



two hundred, the power of the eye-piece now used in the transit telescope. 

 By means of an electro-magnet, these artificial stars (ten in number) attached 

 to my revolving disk, are made to record the exact moment at which they 

 transit an artificial meridian line. The observer, by the aid of another 

 magnet, records the moment of his observed transit, and the difference of 

 these two records, corrected for difference of armature and gross time of 

 the two magnets, gives me the ' absolute personality of the eye/ This same 

 quantity has been obtained also from a record of the moment at which the 

 eye perceives a white ' stripe ' on a dark ground, thrown into view by the 

 sudden action of the electro-magnetic armature, which records the moment 

 of transit of the artificial stars. These quantities, as will be seen hereafter, 

 are almost identical. To obtain ' absolute personality of the ear/ the ob- 

 server, with his magnetic key, attempts to record the moment he perceives 

 the sound produced by the fall of the ' time-pen ' on the disk, as driven by 

 the armature of the time electro-magnet, it falls, and makes its ' true ' dot. 

 The interval between the dot struck by the time-pen and that struck by the 

 observer's recording-pen, gives the time required for the ear to execute its 

 office, and for the nerves obedient to the will to execute the record. A like 

 process, which is quite unnecessary to describe, gives ' personality of the 

 touch/ To the practical astronomer the personality of eye and ear are 

 alone important; to those who have adopted the American method of tran- 

 sits, the ' personality of the eye ' remains as the quantity whose value and 

 variations it is required to determine. Our regular observations have been 

 continued daily, with few exceptions, through some sixty or seventy days, 

 my assistant, Mr. H. Twitchell, and myself making an equal number of 

 observations to determine the following quantities : 1. Absolute personality 

 of eye. 2. Absolute personality of ear. 3. Observed moment of transit. 

 4. Observed moment of emersion. 5. Observed moment of immersion. 

 These constitute the regular observations. Besides these, I have obtained 

 the 'eye and ear personality' of about thirty persons (not observers) of each 

 sex, and of ages from fourteen to seventy-five years. Among these individ- 

 uals I find thus far no law which seems to govern the personality. The 

 range is small, as the personality of eye varies between the lowest limit 

 137 thousandths of a second, and the highest limit 214 thousandths of one 

 second of time. The personality of ear has for its least amount 137 thou- 

 sandths of a second, and for its greatest limit, 223 thousandths of one second 

 of time; and each of these limits belongs to the same two observers. The 

 mean personality of my own eye, as obtained from 255 observations, is 161 

 thousandths of a second. The mean personality of my ear, as obtained 

 from an equal number of measures, is 1G4 thousandths of a second. These 

 same quantities for Mr. Twitchell, as given by the sjime number of observa- 

 tions on the same days with my own, arc for the eye 144, and for the ear, 

 153 thousandths of one second of time. My minimum ' eye personality ' is 

 139 thousandths of a second, the maximum reaches to 191 thousandths. 

 My minimum ' ear personality ' is 143, my maximum ' ear personality ' is 

 193 thousandths of one second of time. The same quantities for Mr. 



Twitchell are for ' 



s s 



The eye, minimum 118 maximum 184 



ear " 0129 t( 0201 



Having reached the above results, I was now curious to learn whether the 

 eye and ear were steady for very short periods of time. For this piu'pose 



