396 ERECT VISION. 



is exposed. This duration of impressions is commonly estimated at 

 about one third of a second. It is a phenomenon analogous to that of the 

 continuance of sound in the ear, and subserves an important purpose of 

 keeping vision continuous and distinct during the winking of the eyelids. 

 Commonly it is illustrated by referring to the familiar experiment of a 

 stick lighted at one end and twirled rapidly round, which gives rise to 

 the appearance of a continuous fiery circle. Many ingenious and interest- 

 ing toys, such as the thaumatrope or wonder-turner, act on this principle. 



When the eye, particularly after a period of repose, as when we first 

 Ocular spectra wake in the morning, is turned to the window or some bright 

 %^t, an( ^ ^ en c l se d> a spectral impression is for a long 

 time presented to the mind. If, instead of closing the eyes 

 after looking at a bright light, they are directed to some white surface, a 

 dark spectral appearance of the luminous object is seen. The explana- 

 tion of this is evidently that those parts of the retina which have just 

 undergone change are less fit to be acted upon by the more moderate 

 light to which they are now exposed than those which have hitherto been 

 unaffected. Under similar circumstances arise what are termed comple- 

 mentary colors. Thus, if we intently regard a red wafer on which the 

 sun-rays are brightly shining, and then turn our eyes away to a feebly 

 illuminated white wall, a green spectre of the wafer will be seen ; and so 

 of other colors. The complementary tint is that color which, added to 

 the original one, forms white light. The explanation of these colored 

 spectra depends upon the principle just mentioned. 



There have been few optical problems more warmly contested than 

 that of erect vision. The image at the bottom of the eye is 



Erect vision. . 11. t n , 



inverted, but we see the object upright. Some have supposed 

 that we really see things upside down, but have learned to correct the 

 error by the sense of touch. Doubtless the true explanation is to be 

 found in the anatomical construction of the eye. It should be borne in 

 mind that there is a very wide difference between the image formed at 

 the bottom of an eye as we look at it, and, if such an expression may be 

 used, as the eye itself looks at it. We see it from behind, the retina 

 sees it from the front. Or, to put the statement perhaps more clearly, 

 it is one thing to look at the images on the ground glass of a camera ob- 

 scura from behind the instrument, and another to see them, as it were, 

 from the interior of the box. The two positions are upon the opposite 

 sides of a vertical axis, round which we may consider that we have turn- 

 ed, and hence the lateral inversion is corrected. That portion of the im- 

 age which, seen from behind, was on the right of the spectator, is on his 

 left if seen in front. A similar event must ensue in the case of the ret- 

 ina. As we have seen, it is its posterior face, looking at the black pig- 

 ment, which is its sensitive surface. It sees, as it were, looking back- 



