194 ON THE SENSES. 



dwell too long on details, we will illustrate tliis by an example. Suppose a 

 circular object before us. If it is small, so that we can survey it witb the eye 

 unmoved, we at once perceive its form through the sense of relative position ex- 

 erted by the retina ; or, in other words, from the circumstance that the points of 

 the retina occupied by its image lie in a circular line. But if it is so large that 

 we cannot embrace it in one view, we proceed as follows : We so direct our eye 

 that first the image of its uppermost point shall fall upon the central point of 

 the retina; we then move the eye so that the image of the following point shall 

 fall upon the same place, and so on, till one after another the images of all the 

 single and successive points of the ring have made their impression upon the 

 same sensitive portion of the retina. That in this process the impressed place 

 of the retina has remained, the same, we learn from the continuance of the local 

 coloring of the sensations; that the object has a circular form, we gather from 

 the muscular feelings which have accompanied the movement of the eye, and 

 which we have learned to refer to a circular movement. Since these muscular 

 feelings inform us also of the magnitude of the movement executed, we form 

 therefrom a judgment respecting the size of the observed circle. If we would 

 estimate the distance of two observed points from one another, we either enu- 

 merate, in the manner heretofore explained, the number of the mosaic elements 

 of the retina, which remain unaffected between their images, or we direct ouv 

 eyes so that first the image of one point shall strike the middle of the retina, 

 and then move the eye so far that the image of the second point shall strike 

 upon the same place, and noAv compute the interval between the two from the 

 muscular feeling which attended the executed movement and which assists us 

 to form an idea of their direction, size, &c. 



We might draw out these remarks to greater length, and initiate our reader, 

 if he had the patience longer to accompany us, still more deeply in the mys- 

 teries of the eye ; but we limit ourselves to what has been said, because it has 

 seemed to us, under the circumstances, a duty to restrict ourselves to what is of 

 most importance, as well as because many other points would appear to offer too 

 much difficulty to be made either intelligible or interesting to the general reader. 

 There are certain limits beyond which the popular exposition of the natural 

 sciences is impossible, and must partake more or less of the character of char- 

 latanism. To him who has followed us we hope to have sufficiently shown that 

 it needs not a glance towards the inconceivably distant worlds of the nocturnal 

 heavens to be sunk in devout astonishment before the marvels of creation, but 

 that the smallest organ of our bodily frame offers a subject of still more wonder, 

 and, because more accessible to our understanding, of a yet higher degree of 

 interest. 



