Sheet Lightning. 89 



The instrument required for this purpose is by no means com- 

 plicated. It must be composed of a tube from twelve to sixteen 

 inches long, which must have at its further extremity a circular 

 aperture of about a line in diameter. This aperture must be sup- 

 plied with a lens of rock crystal, with parallel faces, and about 

 two lines thick, cut perpendicularly to the edges of a hexahe- 

 dral prism of rock crystal. At the other extremity of the tube, 

 to which the eye is to be applied, there must be fitted a prism 

 of the carbonate of lime, of quartz, or some other crystal which 

 is endowed with the property of double refraction. The prism 

 must also be achromatic. If, without the prism, you direct 

 the tube to a radiating object, or to one simply illuminated, 

 you will see only a circular disk more or less luminous. 

 Through the doubly refracting prism, on the other hand, yoii 

 will perceive two such disks. 



When the light of the object observed is direct white light, 

 the two disks will appear white. If, on the contrary, the illu- 

 minating light reaches the tube only after having been reflected 

 at an angle notably different from 90°, the two disks will be 

 found diversely coloured. Suppose, for example, that the one 

 is red, the other will be green. The tints moreover will be al- 

 tered when the tube is turned ; but they will always be the 

 complements of each other, and their union will produce white. 

 The light which is reflected by the atmospheric air possesses in 

 this instrument all the properties of that Avhich is reflected by 

 glass, water, &c. Thus, if you direct the tube towards the 

 azure sky, you will perceive that the two disks sparkle with 

 the brightest colours. There is only a very narrow zone in the 

 immediate neighbourhood of the sun, and a still more circum- 

 scribed space situated in an opposite direction, where the co- 

 loration may not be ])erceptible. 



After these details, it is scarcely necessary to add a won! 

 in pointing out how this simple tube will conduce to the so- 

 lution of the problem before us. It is now night, the sky is 

 serene, and from time to time sheet lightning illuminates the 

 heavens. After having directed the tube towards the quarter 

 •where the phenomenon most frequently occurs, we attentively 

 look through it as if it were a common telescope. When a flash 

 occurs we immediately perceive in the tube two brilliant disks. 



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