] 74? On the Action of various Lights upon the Retina. 



When the eyes are exposed to strong lights, objects cannot be 

 seen of their true colours, and even lights of ordinary intensity 

 produce a decided deterioration in the tints of a fine pic- 

 ture. Hence it is that we see paintings to most advantage 

 when we view them through two blackened tubes held close 

 to the eye. By this means the colours are not only more bril- 

 liant, but faint lights are brought out which would otherwise 

 have been overpowered by the action of lateral light upon the 

 retina. If we turn a picture upside-down, and look at it with 

 the head inverted, a similar effect is produced, because the 

 image is received upon a part of the retina which is not so 

 frequently used; and it is for the same reason that the colours 

 of the sky and of the landscape near the horizon are so beau- 

 tifully seen by looking at them either between the legs, or be- 

 neath the arm with the head inverted. 



It is well known that the human complexion is seen to 

 greater advantage in candle- than in day-light, unless the com- 

 plexions are very ruddy. This arises from there being so 

 much more red in candle- than in day-light. There are cer- 

 tain states, indeed, of the atmosphere, when dark blue clouds 

 prevail, in which the ordinary complexion appears to great 

 disadvantage; and persons in variable health are often de- 

 scribed as looking ill, when the change arises from the prevail- 

 ing colour of the clouds. 



When gas-lights were first introduced, it was a common 

 complaint among those who frequented the theatre, that they 

 injured the personal appearance of the audience. This bad 

 quality made them so unpopular, that a red colour was com- 

 municated to the light by inclosing it in a reddish-coloured 

 glass. The effect, however, arose from the great quantity of 

 light which was used, and from its influence upon the retina; 

 and if the same intensity of light had been obtained either 

 from oil or from candles, the same effect would have been 

 produced. Our eyes are now so much accustomed to the use 

 of strong lights, that the retina is not so easily rendered in- 

 sensible to the red rays, and the blue colour of the light is no 

 longer complained of. It is, however, still observed, by those 

 who have been for the first time exposed to gas illumination, — 

 and the eyes of such persons must therefore serve an appren- 

 ticeship before they learn to see objects in their true colours. 



The blue colour of gas-light was ascribed to the bad- 

 ness of the gas ; and the apparent removal of this injurious 

 quality has been attributed to its increased purity, and to 

 improved methods of burning it : but the truth is, that bad 

 gas, or an imperfect combustion of good gas, produces a 

 much redder light than good gas burnt in the best manner. 



