62 
POPULAE SCIENCE EEVIEIV. 
organs ; there is in each organ also a receiving nervous surface ; 
there is from this surface, leading unto the man, a communi- 
cating nervous cord ; and, at last, ending the communicating 
cord is a nervous centre, in ready communication with 
a congeries of nervous centres, for taking up the impres- 
sion conveyed, for fixing it, and for bringing it into 
union with other impressions that have already been re- 
ceived, fixed, associated. Suppose all these parts at all times 
natural, at all times in harmony, then everything that seems 
unnatural would be fairly ascribed to the reception of actual 
outward manifestations that are not of the common denomina- 
tion of nature. Suppose, on the other hand, that these parts 
are not always in harmonious working order, then the design 
unfolds itself that there may be impressions, made by or within 
the man, that are mysterious, unreal in so far as the true 
reading of the outer universe is concerned, and, in a word, 
hallucinator}^. 
And this is what physical science teaches, that each of the 
parts named as factors are, at times, disturbed or deranged in 
function. The collecting organ may be at fault ; the receiving* 
nervous surface may be at fault ; the communicating cord may 
be at fault ; the receiving centre may be at fault : and, in 
accordance with error of function in one or more of the parts, 
there will be aberration varying from that which is simply phy- 
sical to that which is psychological in the most refined degree. 
The simplest illustration of derangements of function are 
met with when there is perversion of action in the collecting 
organ, as in the eye, in instances of colour blindness or of 
muscae volitantes — floating specks appearing in the field of 
vision. More complex, is a condition in which the reception of 
an impression on the receiving nervous surface of an organ of 
sense is too long retained, so tliat the impression remains when 
the first cause of it is gone. 
Sir Isaac Newton, looking too intently at the sun, had 
left upon his vision a vivid picture of the sun, a phantom to 
some men, to him a phenomenon, painfully persistent, but un- 
derstood by him as a pure physical fact. I knew once a gentle- 
man who had a peculiar impression of an odour left on his 
olfactory surface, and for months it remained a source of con- 
stant discomfort, anxiety, and even timidity. In vision an 
aberration of function in the receiving surface may occur from 
mere strain to see in obscurity. Thus in looking at an object 
in partial darkness, as at night, when the stars are beclouded, 
an object, steadily and strainingly gazed at, seems to come and 
go, or, as is commonly said, to vanish and reappear. 
There are various states of the nervous organization in which 
the conduction of external impressions from the organs of 
