250 SUBJECTIVE FUNCTIONS OF THE EYE. 



moment when this nucleus has shrunk to a mere point 

 the luminous circle breaks out suddenly again, of the 

 same size and aspect ; and the spectral series is re- 

 peated precisely in the same order and time as before, 

 four or five such series occurring within a minute. 

 Gradually the phenomena become feebler ; and I 

 have generally found that after a certain time, perhaps 

 half-an-hour, they wholly disappear. I have never 

 known them recur after the first period of sleep. 



The particular posture in bed does not alter the 

 appearances. And I may further add that they occur 

 equally in any locality, on sea as on land. 1 



I have used here the phrase of ' directing attention 

 to the organ, and I revert to this, inasmuch as it ex- 

 presses, thus applied, a great physical fact not suffi- 

 ciently recognised, viz., the power the mind has of 

 momentarily attaching its perception or consciousness 

 to particular portions of the body, and ever with some 

 change of feeling or function thereby produced in the 

 part. In a chapter of my ' Mental Physiology ' I have 

 dealt expressly with this subject ; but I could add 

 much more upon it were this chapter to be written 

 again. 



1 In a lecture delivered at Leeds some years ago, Sir J. Herschel 

 describes various analogous ocular spectra occurring to himself ' under 

 forms of symmetry and geometrical regularity ' some of them circular 

 and singularly like those I have described above. I do not recollect, 

 however, that he denotes their intermission and recurrence in exact 

 similarity of interval and aspects. 



