35 1 Observations on the Visibility of the Retina ; 



which reflects white light from all its elevated parts. When 

 the eye has been so fatigued that it would see a bright acci- 

 dental green, bring a candle close to the excited eye, and 

 so near its axis that the red seal will be seen by rays which 

 pass near the flame of the candle. When this is done, the red 

 wax seal will be converted apparently into a seal of black 

 wax, the lights reflected from its elevations being still di- 

 stinctly seen. This experiment, when successfully made, af- 

 fords one of the most remarkable optical deceptions with 

 which I am acquainted. 



The method now described of eliminating the impression 

 of the primitive or exciting colour leads us to a very import- 

 ant determination in the theory of accidental colours. I en- 

 deavoured long ago to show from analogy, as well as from 

 the evidence of experiment, that the vision of the primitive 

 and the accidental colour is contemporaneous, in the same 

 manner as the fundamental and the harmonic sound are heard 

 contemporaneously by the ear. That this is the case may be 

 shown in the following manner. When the eye is fatigued 

 with the excitation of the red seal, a faint green phosphore- 

 scent-looking light covers for a while the surface of the red 

 seal, occasionally overpassing its margin, showing, in the 

 clearest manner, that the accidental green is seen at the same 

 time with the exciting red. The effect of this vision of the 

 green is to make the red appear much paler by its admixture 

 with it. The red and green tend to produce whiteness ; but 

 as the direct red greatly predominates over the accidental 

 green, the result is always a pale red. But when a brilliant 

 light is brought near the excited eye so as to extinguish com- 

 pletely the red rays, the phosphorescent green appears alone; 

 and thus we have ocular demonstration that the accidental 

 green is not the light of a white ground deprived of the red 

 rays to which the eye has been rendered insensible, but is a 

 colorific impression generated in the retina itself, and super- 

 added to the whiteness of the ground in the case when the 

 eye is turned from the exciting colour to a white object. These 

 results are obviously incompatible with the theory of acci- 

 dental colours recently published by M. Plateau. 



LXI. Observations on the Visibility of the Ttetina; with lie- 

 marks upon its probable Cause. By T. W. W.* 



r T , HE remarkable experiment in which the blood-vessels of 

 ■*■ the retina are rendered visible has excited so much at- 

 tention, that the following observations upon it may, perhaps, 



* Communicated by the Author. For other papers on this subject see 

 Lond. and Edinb. Phil. Mag. for January, February, and April. 



