with Hematics upon its probable Cause. 355 



not be thought undeserving of a place in the pages of the 

 Philosophical Magazine. It should be previously stated that 

 the writer is so near-sighted as not to be able to see any ob- 

 ject with perfect distinctness at a greater distance from the 

 eye than 8 or 9 inches : but this defect has not proved in the 

 slightest degree prejudicial to the inquiry in the present case, 

 being associated with a power of enduring a glare of light, 

 which has permitted the experiment to be repeated many 

 times under the most favourable circumstances. 



The ramifications of the blood-vessels are seen by the 

 writer without difficulty, whenever a motion in any direction 

 is given to the flame of a candle at the distance of a few inches 

 from the eye, nor do they instantaneously vanish when the 

 motion is discontinued. They invariably appear very dark, 

 upon a brownish ground. The position of the flame with 

 respect to the eye is not material, so long as a distinct image 

 of it is received upon the retina. In proportion as the candle 

 is removed from the axis of vision, the ground appears darker 

 upon which the vessels are seen, while they become more 

 sharply defined, their distinctness seeming to depend upon 

 their distance from the flame. The effect is precisely the 

 same whether the eye or the candle be moved. The motion 

 of the flame, the eye remaining at rest, produces an apparent 

 motion of the spectrum, always opposite in direction, but 

 much inferior in degree. When two flames are alternately 

 employed in rapid succession, one in front of the bridge of 

 the nose, the other near the exterior canthus, the vessels suc- 

 cessively observed appear at first not to be the same ; but this 

 is merely a deception, arising from a difference in the position 

 of the spectrum corresponding to the change in the direction 

 of the light. When two flames are placed on the same side 

 of the eye, two spectra, separated from each other by a small 

 interval, have occasionally been seen, but with difficulty. 

 When two flames are simultaneously employed on opposite 

 sides of the eye, and moved in contrary directions, the two 

 spectra may be plainly observed at once, and easily distin- 

 guished from each other by their relative motion. The ad- 

 dition of a second candle doubles or at least materially in- 

 creases the brightness of the ground upon which the vessels 

 are seen. The interposition of a concave lens of 8 inches 

 focus, which is requisite to produce perfect vision of distant 

 objects, considerably diminishes the distinctness of the phe- 

 nomenon. When a convex lens, of about an inch focus, is 

 so placed with respect to a candle that its whole area appears 

 luminous, the candle also being very near the eye in an ob- 

 lique position, the blood-vessels are seen stronglv marked and 

 2Z2 



