SENSE ORGANS. 



127 



the retinal surface. Or it may be otherwise expressed 

 thus : The impression produced by light from any radiant 

 point in space is 

 referred back along 

 the ray line to the 

 place whence it 

 came. Either of 

 these formulae is 

 sufficient for gen- 

 eral statement, 

 but the former is 

 probably the more 

 exact. The lat- b 

 ter, however, we a'' 

 will use for illus- 

 tration. Thus we 

 have two con- 

 caves, the spatial 

 and the retinal ; 

 two worlds, a 

 macrocosm and a 

 microcosm ; and 

 these correspond 

 with one another 

 point for point, 

 and exchange 



with one another by impression and by external refer- 

 ence along the lines connecting them. This is repre- 

 sented in Fig. 87. In this figure, of all the rays proceed- 

 ing from a radiant point and gathered by the lens to a 

 focal point on the retina, we take only the central ray, 

 for this represents the resultant impression. These cen- 

 tral rays cross one another at the nodal point n. If now 

 a, b, e, be three stars, then the light from a impresses 

 the retina at a', and is referred straight back along the 



Fig. 87. — Diagram representing corresponding 

 points, retinal and spatial. 



