POWER OF SHORT SPACES. 31 



clently distant, it is similar to the luminous body. The in- 

 terstices between the leaves of trees are so many small irregular 

 apertures ; hence the cause of the numerous small bright circles 

 seen in a sunny day in the shadow of a tree, or still more dis- 

 tinctly in that of a grove,"* 



These simple laws which govern the projection of shadows, 

 and which have been seen to adapt themselves to individual 

 cases, may be easily verified. It is by their judicious combi- 

 nation, however, that we discover the best method of throwing 

 large and well-defined images of small, near objects upon the 

 bottom of the eye, which indeed constitutes the main design 

 of our inquiry. Thus of the three modes of illuminating the 

 object which have been enumerated, that is obviously the best 

 suited to our purpose which casts the broadest shadow. A 

 divergent pencil of light is therefore chosen. In the next place 

 he position of the screen demands attention, for on this, as we 

 have seen, depends the definition as well as the enlargement 

 of the image. Now in the investigation of small, near bodies, 

 the screen cannot possibly be brought too close to the eye ; in- 

 deed it is better to dispense with all artificial substitutes, and 

 to use that kind of screen only which nature has provided. 

 That is to say, the retina of the eye itself. This has accord- 

 ingly been adopted. 



Again, recollecting the impossibility of distinguishing out- 

 line at all, except by contrast, — a mass of shade bounded by 

 light, or light by shade, — definite contiguous portions of the 

 retina are simultaneously affected with such impressions by 

 using a darkened tube to exclude the light, having small inlets 

 of determined size to regulate its admission at one end, and 

 openings to secure its transmission and exit at the other. In 

 this way, light and dark spaces are brought into direct conti- 

 guity with a well-defined line of demarcation between them. 

 Thus small objects are appreciable. 



But, moreover, a shadow, like its substance, appears larger 

 as it approaches the eye ; and the amount of enlargement is 

 regulated by the same law. Hence the one is equivalent in 

 this respect to the other : and as a shadow can be projected 

 directly in front of the eye, and received as an image, it is 

 thereby much magnified ; nevertheless at such short distances, 

 both shadow and substance, by any other process, would prove 

 invisible. 



Here, then, we have within our reach the combination of 



elements which appear necessary for examining small objects, 



at very short distances from the eye ; namely : — A darkened 



retina, a diminutive object less than the pupillary aperture, held 



* See Chambers' ' Optics,' p. 14. 



