On the Halo seen around all Bodies. 31 



mey appear in their true place, though not always of their true 

 size, for this depends on the diameter of the pin-hole. There is 

 however another very important circumstance which contributes 

 to the erect position of external objects — the axis of the piti-hole 

 and our eye are strictly "parallel^ and all the rays which pass 

 through the centre of this axis carry an erect impression of the 

 object whence they emanate. 



43. I observed above that when the pin-hole is near the eye, 

 its lenticular property is apparent. If we hold a pin between the 

 eye and the pin-hole, the pin will be inverted. The reason of 

 this is obvious, for according to the 23d section, the halo is len- 

 ticular in the direction of its parallelism with the edge or surface 

 of the body on which it rests. If the pin-hole were oblong, or 

 if the aperture were a long slit instead of a round hole, the sha- 

 dow of any slender object before the eye would remain the same 

 as to position, but be reversed as to movements. But the pin- 

 hole being a perfect lens — for it is completely filled by the halo — 

 any shadow cast on its surface will be reversed both in position 

 and movement. The character of these halos never varies ; they 

 act upon one another in a sensible manner and always produce 

 the same results. 



44. Should a doubt remain as to the lenticularity of this halo 

 on the pin-hole, we have only to observe the figure which ap- 

 pears on its surface. On looking steadily at the pin-hole we shall 

 perceive that the rays of light from a candle, and which are 

 brought to a focus on the lens of our own eye, give to the cere- 

 bral organs of vision an exact representation of all specks, flaws, 

 spots, and movements of fluids which actually exist within the 

 eyeball. I say cerebral organs, because a true knowledge of vis- 

 ion must convince us that the figure which appears to be in the 

 pin-hole cannot by any possibility be painted or impressed there, 

 and if not there, certainly not on any part of the interior of the 

 eyeball. The very circumstance of being able to see any part 

 of the interior of our own eye should settle the question of the 

 seat of vision, and prove that the eyeball and its internal appara- 

 tus are merely for the purpose of transmitting rays of light and 

 not images ; that the images which are the result of these rays, 

 are impressed on the cerebral organs with thej^rs^ touch of light 

 on the elastic machinery of the eye, and never come to a focus 

 on the retina or on any other part of the interior of the eyeball. 

 But I have pursued this branch of the subject elsewhere. 



