260 PERSONAL PREPARATION FOR RESEARCH. 



unless all those conditions are present, perfect sleep does 

 not occur. Most of the^circumstances favourable to sleep 

 have already been mentioned. 1 Absence of internal 

 and external sensation is not alone sufficient to secure it ; 

 equally essential is absence of memory of optic images, 

 because the eye is the most intellectual of all the senses. 

 Absence of haunting ideas is also a necessary prelimi- 

 nary ; this may usually be secured by making a written 

 memorandum of such ideas, and by allowing the mind- 

 to be occupied by uninteresting monotonous or desultory 

 ones requiring but little exercise of the attention. Any- 

 thing which requires much attention or volition tends 

 to postpone sleep, until by the very act of attention the 

 brain becomes quite exhausted. 



As also the activity of the mind is intimately depen- 

 dent upon the oxidation of nervous matter by the oxygen 

 in^he blood passing through the cerebrum, and the amount 

 of this oxygen is less, and that of carbonic acid is greater 

 in the blood during slow respiration and sleep, slow respira- 

 tion, and the inhalation of the portion of the carbonic acid 

 in the products of respiration, help to produce sleep. 2 It 

 is well known that a monotonous uninteresting discourse, 

 listened to in an atmosphere highly laden with the pro- 

 ducts of respiration, has a soporific effect. Warmth of the 

 extremities also, by withdrawing blood from the brain, 

 produces a similar result. Upon the general principle 

 that the motion of a conducting body is retarded in a 

 magnetic field, powerful magnetism might be expected to 

 retard the circulation of blood in the brain and promote 

 sleep. 



The religious opinions of men appear to have very little 

 influence upon their power of making scientific discoveries ; 



1 See p. 39. 



2 See Mind, vol. ii. part viii. p. 571. October, 1877. 



