248 ANIMAL PH73IOLOGY. 



be much, greater, were ifc not that the eye has a tendency to 

 follow any object on which the gaze is fixed. The eye follows 

 the wheel of a carriage, and the image continuing to be made 

 on one part of the retina is correctly appreciated as circular ; 

 but it fails to follow the spokes, and therefore these affect 

 successive parts of the retina with great rapidity. The dura- 

 tion, of retinal impressions is the principle on which a number 

 of optical toys depend. The simplest of them are cards with 

 pictures on each side, and twirled round by a couple of 

 strings, one at each end, so as to bring the pictures together. 

 Thus, a bird on one side and a cage on the other gives the 

 appearance of a bird within a cage ; and a man riding a horse 

 may be brought into view, the man being painted on one 

 side of the card arid the horse on the other. The thaumotrope 

 and the anorthoscope are instances of much more complex 

 toys dependent 011 the same principle (see Glossary). 



The coloured spectra seen after gazing on bright objects are 

 connected with something more than the mere duration of 

 impressions, namely, the power of appreciating different 

 colours. The appreciation of colour is not understood; we 

 do not know by what mechanism the rods and cones are 

 thrown into different conditions by lights of equal intensity 

 but different wave lengths, so that both colour and intensity 

 are appreciated by one set of structures. But it is known 

 that the appreciation of colour is a separate power from the 

 appreciation of light and shade, for there are various kinds of 

 colour-blindness which occur uncomplicated with other defect 

 of sight. 



184. Colour-blindness is of three sorts. Some persons have 

 been found unable to appreciate any difference of colour at all, 

 to whom the world was like an engraving. A number of persons 

 have an inability to distinguish allied tints one from another, 

 and confuse blue with green, or confound pinks, crimsons, 

 and scarlets together. In a third set of cases, colours totally 

 unlike are confounded; and the most remarkable colour- 

 blindness of this description consists in inability to distin- 

 guish red from green or blue. Curiously enoiigh, this defect 

 may exist without the object of it having any suspicion that 

 his vision is defective. Thus, Dalton, the celebrated chemist, 

 from whom colour-blindness occasionally gets the name of 



