hy the Action of Luminous Points and Lines. 173 



never been explained. When a spectral impression of any 

 very luminous body has become so weak that it can no longer 

 be seen on a white ground, it is instantly revived by shutting 

 the eye, and continues to be seen for a short time when the 

 eye is again opened. The obliteration of the spectral image 

 arises from the white light around it, extending its influence 

 to the part of the retina occupied by the image ; and the mo- 

 ment this action is removed by shutting out the light the ori- 

 ginal impression is revived, or rather is rendered visible by 

 the removal of another impression which overpowered it. 



Connected with these views is a very remarkable experi- 

 ment described by Dr. Purkinje of Breslau, and which has 

 been communicated to me by Mr. Potter, who has frequently 

 and successfully repeated it. — If a candle is held a foot or two 

 before one eye in a room without any other light, and is viewed 

 directly by the observer, a mass of reddish-brown light is seen 

 around the candle, and on this light, as a ground, are seen the 

 ramifying blood-vessels of the retina, the base of the optic 

 nerve and the foramen centrale. Mr. Potter finds that this 

 experiment succeeds best when the candle is held about a 

 foot from the eye, making an angle of about 20° with the line 

 of distinct vision. I have tried it repeatedly, and under all 

 forms, but I can see nothing excepting the mass of brown 

 light. The prevailing explanation of this curious fact is, that 

 the light which surrounds the candle is reflected back upon 

 the retina, either by the inner concave surface of the crystal- 

 line lens, or of the cornea; and that the objects are, somehow 

 or other, magnified by these concave surfaces. The moment 

 I repeated this experiment, I recognised in the mass of nebu- 

 lous reddish light the very same phaenomenon which I had 

 long before described as seen round luminous objects viewed 

 indirectly. I have no doubt, therefore, that this light is pro- 

 pagated from the luminous image of the candle, and that 

 though the retina, in contact with the blood-vessels, is sensi- 

 ble to direct light, it is insensible to propagated light, and 

 therefore the blood-vessels must be delineated in obscure lines. 

 As there is no retina across the foramen centrale, it will of 

 course appear as a black spot ; and owing to the obtuse vision 

 of the optic nerve, it will appear less luminous than the sur- 

 rounding retina. 



In referring to the phaenomena of indirect vision, I cannot 

 avoid noticing the fact, that a candle seen by continued in- 

 direct vision appears more luminous than one seen directly. 

 This led me to conceive the idea that it might be possible 

 to generate, as it were, light by increasing its physiological 

 action on the retina. Whenever we condense light for occo- 



