with Remarks upon its probable Cause. 357 



zontal direction, the ratio of the axes being by estimation 

 about as 4 to 5. When the flame is sufficiently removed from 

 the axis of vision in any direction, not a trace of the rim or 

 shadow can be perceived. That part of the foramen which is 

 not actually in shadow does not in any case appear darker 

 than the surrounding retina; when brought very near the 

 flame it hecomes brighter, and an indistinct yellow glare is per- 

 ceived without it, on the side furthest from the light, such as 

 might result from reflection from the interior of a slight de- 

 pression; when very near the flame, this reflection appears to 

 extend considerably beyond the limits of the foramen. In 

 certain positions, especially immediately above the summit of 

 the flame, some minute ramifications of vessels may be traced 

 within the foramen, on its apparently upper side. 



Such are the phsenomena observed when the experiment is 

 tried with a flame from 4 to 6 inches distant from the eye : 

 when it is removed to twice the latter distance, the ramifica- 

 tions, though still visible, are less distinct, and upon a darker 

 ground ; they ma)', however, be more plainly detected within 

 the foramen than before. At the distance of 18 inches the ra- 

 mifications disappear ; but with great attention the foramen 

 may be discerned, and the diminished glare of light permits 

 a more careful examination of it. "When about twice its own 

 longest diameter removed from the flame, it appears filled 

 with shadow ; but as the interval diminishes, the shadow is 

 succeeded by a yellowish light, brightest in the part which is 

 most usually in shade, namely, the declivity of the sides: this 

 effect does not extend across the centre of the foramen, which 

 is occupied, even when nearly in apparent contact with the 

 flame, by an ill-defined dusky blot. The rim which sur- 

 rounds the cavity is very visible in the same situation, on the 

 side next the candle; but it is now converted into a dark line, 

 forming a conspicuous object in the greatest possible proxi- 

 mity to the flame. 



When the experiment is repeated with the solar rays, the 

 eye being gently moved through a small space, or the 

 rays being alternately admitted and intercepted, the phac- 

 nomenon may be very distinctly seen, but not so conveni- 

 ently as with an artificial flame ; the direction also of the 

 light is required to be more oblique, or the spectrum is effaced 

 by its brilliancy. The blood-vessels occasionally exhibit an 

 indistinct effect of light and shade, as in certain cases with the 

 flame of a candle. The ridge of the nose being made by a 

 slight motion of the head to admit and exclude alternately 

 the beam of light, the ramifications become successively dark 

 and bright without vanishing, at least in the left eye. In this 



