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On Natural Magic. 259 
sence, or partially by the mitigation, of this action. When therefore 
one keeps his eyes for a time directed to a portion of black surface 
surrounded by white, the sensibility of all that part of the retina on 
which the white surface throws its light, is weakened in a much 
higher degree than that which is occupied by the image of the black 
' portion. Then on turning off the eyes to a quarter from which 
light comes nearly uniform, the effect on this now most sensitive 
portion, is contrasted with the slighter effect produced on the sur- 
rounding parts, and there appears to the observer, as it were, an 
image of light, in shape and size like the portion of black surface 
before viewed. 
Now the relative amount of light reflected from white and from 
adjacent dark surfaces, is probably the same, whether the incident 
light be feeble or strong, and consequently the relative strength of 
their respective impressions on the retina is alsothe same. And in- 
deed the eye, especially if it has been for some time previously in 
the dark, seems to be not less sensible to this difference of impres- 
sion in a twilight than at noonday, provided the darkness be not too 
great, so as to render all objects nearly alike obscure, But how- 
ever this may be, the appearance of ocular spectra in such fainter 
light, is favored by the fact that the attention does not then, owing 
to the partial obscurity in which the substantial objects before us lie, 
so readily and so almost unavoidably fix itself upon them, which 
if it should do, any image that may remain impressed on the retina 
is not regarded, for the mind it seems cannot attend to two things at 
the same time. Another reason why such phenomena are so sel- 
dom noticed by individuals who do not purposely take the prelimi- 
nary steps necessary to produce them, is that the eye is usually a 
_ restless organ, rarely dwelling upon the same part of an object for 
more than a few minutes at atime. The designand effect of this is, 
on a compensating principle, to prevent the formation of any impres- 
sions of such a character as to be inconveniently permanent or embar- 
rassing to our vision. This propensity to wander is, however, some- 
times overcome, and the occasions when this may happen are various. 
A day or two since, listening to a public speaker at such a dis- 
tance, that, to catch his words I found it necessary continually to 
watch his lips, I at length cast a look towards the expanse of white 
ceiling beyond him, and saw a white picture clearly representing 
him, wherever I turned my eyes. The propensity before adverted 
to, is more commonly subdued involuntarily by grief, as for the de- 
