OCULAK SPECTRA. 



The image of this point will, as before, be formed at o, the point 

 where the optical axis D c meets the retina. Let us now suppose 

 the axis of the eye to be turned aside through the angle D c D', 

 the optical centre will then be removed from c' to c", and the 

 image of o will now be formed at the point o", where the line o c" 

 meets the retina. The direction, therefore, in which o will now 

 be seen, will be that of the line c" o, whereas the direction in 

 which it was before seen was that of the line c o. The point of 

 the retina at which the image o was originally formed is removed 

 to o', while the image is removed to o". Thus there is a displace- 

 ment not only of the retina behind the image, but also an absolute 

 displacement of the image, and an absolute change in the apparent 

 direction of the object. Since no such change in the apparent 

 direction is consequent upon the movement of the eye in its socket, 

 it follows that the optical centre c' of the eye must coincide with 

 its geometrical centre c. 



70. The continuance of the effect produced by the image of a 

 visible object on the retina after such object has been removed 

 from before the eye, combined with the effect of the image of 

 another object placed before the eye during such continuance of 

 the effect of that which was removed, produces a class of phe- 

 nomena called ocular spectra and accidental colours. 



The effect produced by a strongly illuminated image formed on 

 the retina does not appear to be merely the continuance of the 

 same perception after the image is removed, but also a certain 

 diminution or deadening of the sensibility of the membrane to 

 other impressions. If the organ were merely affected by the con- 

 tinuance of the perception of the object for a certain time after its 

 removal, the effect of the immediate perception of another object 

 on the retina would be the perception of the mixture of two 

 colours. Thus, if the eye, after contemplating a bright yellow 

 object, were suddenly directed to a similar object of a light red 

 colour, the effect ought to be the perception of an orange colour ; 

 and this perception would continue until the effect of the yellow 

 object on the retina would cease, after which the red object would 

 alone be perceived. 



Thus, for example, a disk of white paper being placed upon a 

 black ground, ana over it a red wafer which will exactly cover it 

 being laid, if, closing one eye, and gazing intently with the other 

 for a few seconds on the red wafer, the red wafer be suddenly 

 removed so as to expose the white surface under it to the eye, 

 the effect ought to be the combination of the perception of 

 red which continues after the removal of the red wafer, with the 

 perception of white which the uncovered surface produces ; 

 and we should consequently expect to see a diluted red disk, 



77 



