﻿thus : s^4y~\* But the pinholes are seen in their true positions, 



268 Prof, J. LeConte on an Optical Illusion, 



paper box (fig. 2), this may be easily shown by making a number 

 of pinholes at a. In each an inverted pinhead will be seen 



■<& 



though the pins are inverted. The reason is plain. The radiant 

 cones from the se- pj 0>- ^ 



veral pinholes cross 

 each other (fig. 3), 

 as all radiant cones 

 do, and their posi- *•'/ 

 tions on the retina 

 are therefore inver- 

 ted; while the rays 

 of each cone do not 

 cross, as rays of ra- 

 diant cones never 



do unless the retina is beyond the focal point ; and the retinal 

 shadows (A, A') of the pin are therefore erect. 



2. Mr. Tupper seems to think that his experiment overthrows 

 the law of visible direction, and draws the arrow B (fig. 1) to 

 show the direction in which, according to this law, the pinhead 

 ought to be seen. But in this I think he is mistaken. The ex- 

 periment confirms the law, as indeed does every phenomenon of 

 vision. The law of visible direction is, that every point of the 

 retinal impression is referred to the field of vision along a line 

 passing through that point and through the optic centre. The 

 visible direction is not represented by the line A B, but by the 

 dotted line A C which I have added. This gives the pin inverted 

 as we actually find it. How Mr. Tupper was led into this mistake 

 I know not* — unless he supposes that the ray A, followed back 

 along its last course, gives the visible direction. In ordinary 

 vision, it is true, the visible direction for each point is the axial 

 ray of the cone followed back ; but this is only because this ray 

 passes through the optic centre. But Mr. Tupper says "the 

 object is seen where it is not." Musci volitantes are also seen 

 where they are not, not only as to distance but also as to direc- 

 tion. Those above the optic axis are seen below the line of sight, 

 and those below are seen above. The law of visible direction 

 gives us the true position of objects seen by images, but not of 

 objects seen by retinal shadows ; but, properly understood, this 

 is one of those apparent exceptions which prove the law. 



3. The last paragraph of Mr. Tupper's letter I do not quite 

 understand; but it certainly contains an erroneous statement. 

 "It may be noticed," he says, "that this analysis demonstrates the 

 peculiar effect of what I call single-ray delineation on the retina, 



