SENSE ORGANS. I2 n 



such reference, the exact place of all objects and radiants 

 in space. Together they explain all the phenomena of 

 monocular vision except the perception of color. With 

 this exception, the whole science of monocular vision 

 is but an explication of these two laws. All that we 

 have further to do, therefore, is to take up several 

 important subjects and show how completely they are 

 explained by these laws. 



I. ERECT VISION. 



Statement of the Problem. — We have seen that 

 the retinal images are inverted. We have also seen that 

 these images are referred, as it were projected, outward 

 into space, and are seen there as external images, the 

 signs and facsimiles of the objects which produced them. 

 How is it, then, that objects are seen in their natural 

 positions — i. e., erect? This problem has puzzled think- 

 ers ever since the inverted retinal image became known. 



True Solution. — But there is no mystery at all 

 about it, if we clearly understand the law of direction. 

 Most reasoners on the subject do not seem to perceive 

 that the problem of erectness of objects does not differ 

 at all from that of seeing objects in their right places. The 

 latter concerns the true position of objects, the former 

 the true position of radiants. Objects are composed 

 wholly of radiant and retinal images of corresponding 

 focal points. Now, if images are referred each back 

 along its ray line to its proper place in space, then also 

 the focal points of these images are similarly referred 

 each to its proper place. Now, as ray lines from radi- 

 ants cross one another at the nodal point and thus in- 

 vert the image, so the reference lines recross at the same 

 point, and thus reinvert the image in the very act of external 

 reference. This is shown in Fig. 86, except that now an 

 object replaces the stars. It is evident that the two ends 



