ox PHTSIOLOGEC.iL LIMITS OF MICROSCOPIC VISIOX. 473 



use of the unarmed eye,) contrast of outline, coloring, relative light 

 and shade, (as in looking at a landscape) is recognised mainly by a 

 mental comparison rapidly effected hy quick alternate direction of the 

 axis of vision towards one boundary or another, from one surface to 

 its next contiguous surface, and so on. And this mode of using the 

 eye, long continued discipline of mental vision alone can teach, 

 until finally the inexactitude of the actual retinal images is so com- 

 pensated and rectified by experience that the adult eye rarely fails 

 of a correct conclusion. The mental vision thus taught and 

 exercised is so different from the retinal impressions (because 

 every imperfect or unnecessary impression is discarded by 

 the mind,) that people believe what they see simply because they see 

 what they believe. I^ow the same training is needed for vision 

 through the microscope. With the monocular instrument sharpness 

 of impression and of direction predominates, whilst the peculiar 

 advantages of vision with two eyes are lost. With the binocular 

 these advantages are regained, but the picture is a composite one 

 with strong perspective effects, though the capacity of appreciating 

 linear direction to the axis of each eye singly is lost. Each 

 instrument has its special uses, but the eye requires training for 

 both, and in all microscope i^Tactice fatiyue of the eye physiologically 

 affects the distinctness of vision. We know that too much light 

 involves fatigue of the retinal perceptive faculty from over intensity 

 of impression. „" /w a single minute the impression produced by a 

 bright surface has lost from a quarter to half of its intensity, and yet 

 the observer does not notice this fact until contrast brings it before 

 him.'^ (See Helmholtz's Popular Lectures, page 225.) Too little 

 light, again, involves fatigue from strain of attention, and consequent 

 tremor of the muscles of the eye. For the definition of delicate 

 outline depends upon trustworthy recognition of subtle gradations 

 of light and shade, and suffers in proportion as the brightest parts of 

 a picture fade. If the absence of light produce a lower tone than is 

 requisite for definition, the mental direction of the movements of 

 the eye for observation of contrasted light and shade is not called 

 into action, or if forced it fatigues still more. If, again, an object 



