244 Dr. Tyndall on the Progress of the Physical Sciences : 



developed by the red glass is bluish-green, and by the blue 

 glass, red. 



Why is it, then, that the red and blue lines cannot be made 

 to combine, but always lie alongside each other crossed in the 

 manner indicated ? M. Dove hnds the explanation in the non- 

 achromatic nature of the eye. That the eye is not achromatic 

 has been known since the time of Fraunhofer ; but a very simple 

 way of proving the fact was discovered independently by M. Dove 

 and M. Plateau about twelve years ago. If the flame of a candle 

 be viewed through a coloured glass which permits the ends of 

 the solar spectrum to pass through it, but extinguishes the middle, 

 at the distance of distinct vision a violet flame is observed. At 

 a greater distance a red flame is observed within a larger blue 

 one which embraces the former on all sides and becomes wider 

 the further we recede from the flame. Within the distance of 

 distinct vision, on the contrary, the violet flame is encompassed 

 by a sharp i-ed rim. From a medium distance a long-sighted 

 eye sees the latter, and a short-sighted eye the former. Hence 

 the experiment furnishes us with a kind of optometer ; to this 

 purpose M. Dove has applied it in hundreds of cases, and never 

 found a single individual whose eyes fulfilled the conditions of 

 achromatism at all distances. Acquainted with this fact, and 

 observing a certain analogy between it and his stereoscopic ex- 

 periments, he naturally sought the cause of the phsenomena pre- 

 sented by the latter in the non-achromatic nature of the eye. 



A fine white line di'awn upon a black ground was viewed 

 through the glasses used in the stereoscopic experiments. It 

 was ascertained that, to be plainly visible, it must be held at a 

 greater distance from the eye when the red glass is used than 

 when the blue glass is applied. Sir David Brewster has ob- 

 tained an analogous result with pigments (Report of the British 

 Association, 1848, p. 48). A number of square pieces of gra- 

 dually decreasing size was cut from the same vividly-coloured 

 card, and placed one upon the other so as to form a pyramid 

 with ascending steps, all of the same height. Two such pyramids 

 were built, the one beside the other ; the squares were blue and 

 red ; one pyramid had a blue square for its base, the other a red 

 one. It was always found that a blue square placed upon a red 

 one appeared higher than a red square placed upon a blue one ; 

 so that in the building of the pyi-amids, each appeared by turns 

 to exceed the other in height. From this experiment it follows, 

 that at the distance of distinct vision the lines of convergence of 

 both eyes enclose a smaller angle in the case of red light than 

 in the case of blue. Hence if an observer, who sees equally well 

 with both eyes, have both colours presented to him in the ste- 



