SENSE ORGANS. I 3I 



We see perfectly the word we look at, but words right 

 and left are increasingly illegible, and therefore we are 

 compelled to run the eye along the line, so that the image 

 of every word falls successively on the fovea. The eye 

 is the most restless of organs. In looking at a scene we 

 sweep the point of sight about and gather up the results 

 in memory, and thus seem to see the whole scene dis- 

 tinctly. But really we see distinctly only a very small 

 area. The explanation of this is doubtless found in 

 retinal structure. As already seen (page 120) the cones 

 of the fovea are connected each with its own fiber, 

 whereas one fiber is connected with several rods. 



Is this limitation of distinct vision a defect ? I think 

 not. Suppose we saw all parts of the field of view with 

 equal distinctness : it would be impossible to fix thoughtful 

 attention on the thing looked at. But the development 

 of the higher faculties of the mind is conditioned on the 

 ability to fix the attention. Thus there are three kinds 

 or grades of vision : 1. Simple seeing, which may be un- 

 conscious and involuntary. 2. Looking, or the volun- 

 tary act of sight. 3. Observing, or the thoughtful, act of 

 sight. This last is characteristic of man alone. 



Minimum Visibile. — What is the limit of sight as to 

 smallness ? We answer, There is nothing so small that 

 it can not be seen if there be light enough. A star is 

 the nearest to a mathematical point that we can well 

 conceive. We may magnify it three thousand diameters, 

 and still it is a point. And yet a star may be seen 

 plainly enough. The only sense in which there is a 

 minimum visibile at all is the smallest thing that can be 

 seen as a magnitude — the smallest distance between two 

 stars that they can be seen as two, the smallest distance 

 between two dots or two lines that they can still be 

 seen as two. This undoubtedly depends on the size of 

 the cones of the fovea. If the images fall on one cone, 



