100 ANNUAL OF SCIENTIFIC DISCOVERY. 



lights shall be fixed wherever it is practicable, so as to exhibit them, and 

 shall be fitted with in-board screens, projecting at least three feet forward 

 from the light, so as to prevent the lights being seen across the bow." The 

 author then went on to show that these regulations, which would effectually 

 secure the object intended in most cases, would be most dangerous, should a 

 seaman be put to steer or look out, who had that peculiar kind of blindness, 

 of which he had encountered mam- instances, of not being able to distinguish 

 red light from green. Statistics of color-blindness are defective, not includ- 

 ing females; but there is reason to think that not less than one in twenty- 

 is defective in this respect; and of the markedly color-blind, not less than 

 ,one in fifty males are so. Out of 1,154 persons, including students, soldiers, 

 and policemen, examined by the author, one in fifty-five were markedly 

 color-blind, i. e. entirely unable to distinguish the colors red, brown, 

 green, and blue. The author suggests tAvo remedies : 1. A change of the 

 system itself, which in its details must be left to nautical men. 2. An 

 examination of all masters, mates, and pilots in the merchant sen-ice, as 

 to their power of distinguishing colored lights within the limits of vision, and 

 rigorously excluding those who could not. 



ON THE DURATION OF LUMINOUS IMPRESSIONS ON CERTAIN 



POINTS OF THE RETINA. 



At the last meeting of the British Association, 1858, Sir David Brewster 

 stated that it was well known that the direction of luminous impressions 

 upon the retina is one third of a second for white light of ordinary intensity. 

 At a former meeting he had shown that the small circular area at the end of 

 the axis, whether it be the retina or the choroid, retains light longer than the 

 general retina, after the eye has been exposed to light ; and he had recently 

 observed that certain points of that membrane, situated, apparently, near its 

 termination at the ciliary process, have even a greater retentive power. In 

 order to observe this curious phenomenon, we must extinguish, suddenly, 

 a gas-flame, to the light of which the eye has been for some time exposed. 

 We shall then observe a number of bright luminous points arranged in a 

 circle, the diameter of which is about 72. These bright points or stars, 

 apparently placed at equal distances, vanish so quickly, that he had found 

 it very difficult to determine then: number. They may amount to fifteen or 

 twenty. He had sometimes observed them upon extinguishing a candle, and 

 also upon quickly shutting the eyes. The parts of the retina from which 

 these points of light emanate, are probably places Avhere the retina is 

 attached to the ciliary ring, or other parts in the interior of the eye, and 

 may, therefore, be detected by the anatomist. 



ON THE LAWS OF COLOR. 



The following is an abstract of a lecture on the above oject, before the 

 Royal Institution, London, by Mr. Grace Calvert. The lecturer stated that 

 he had three objects in view in his discourse. The first was to make known 

 the laws of color, as discovered by his learned master, M. Chevreul; secondly, 

 to explain their importance in a scientific point of view; and, thirdly, their 

 value to arts and manufactures. 



To understand the laws of colors, it is necessary to know the composition 

 of light. Newton was the first person who gave to the world any statement 



