ON visiox, 455 



I- 



by the actual objects: but we may place the picture at such a distance that 

 neither of these criterions can have much power in detecting the Tallacy ; 

 or, by the interposition of a large lens, we may produce nearly the same 

 effects in the rays of light, as if they proceeded from a picture at any requir- 

 ed distance. In the panorama, which has lately been exhibited in many 

 parts of Europe, the effects of natural scenery are very closely imitated : the 

 deception is favoured by the absence of all other visible objects, and by the 

 faintness of the light, which assists in concealing the detects of the repre- 

 sentation, and for which the eye is usually prepared, by being long detained 

 in the dark winding passages, which lead to the place of exhibition. 



The impressions of light on the retina appear to be always in a certain 

 degree permanent, and the more so as the light is stronger; but it is uncer- 

 tain whether the retina possesses this property merely as a solar phosphorus, 

 or in consequence of its peculiar organization. The duration of the impres- 

 sion is generally from one hundredth of a second to half a second, or more; 

 hence a luminous object revolving in a circle makes a lucid rinff; and a 

 shooting star leaves a train of light behind it, which is not always real. If 

 the object is painfully bright, it generally produces a permanent spot, which 

 continues to pass through various changes of colour for some time, without 

 much regularity, and gradually vanishes: this may, however, be considered 

 as a morbid efl'ect. 



When the eye has been fixed on a small object of a bright colour, and is 

 then turned away to a white surface, a faint spot, resembling in form and 

 magnitude the object first viewed, appears on the surface, of a colour oppo- 

 site to the first, that is, of such a colour as would be produced by with- 

 drawing it from white light; thus a red object produces a bluish green spot; 

 and a bluish green object a red spot. The reason of this appearance is pro- 

 bably that the portion of the retina, or of the sensorium, that is affected, has 

 lost a part of its sensibility to the light of that colour, with which it has been 

 impressed, and is more strongly affected by the other constituent parts of 

 the white light, A similar effect is also often produced, when a white,or grey 

 object is viewed on a coloured ground, even without altering the position 

 of the eye: the whole retina being affected by sympathy nearly in the same 

 manner as a part of it was affected in the former case. These appearances 



