PORTION OF THE COSMOS. VISUAL POWER. 45 



blished by Hulagu at Meragha. If looking through tubes 

 facilitates the finding of stars in the twilight,, in other 

 words, if, in the evening twilight/ stars are earlier visible to 

 the naked eye with tubes than without, the reason, as Arago 

 has remarked, is, that the tube, when the eye is kept close to 

 it, cuts off a large portion of the disturbing diffused light 

 (rayons perturbateurs) of the atmospheric strata which in- 

 tervene between the star and the eye. In like manner, even 

 in a dark night, the tube is useful in preventing the lateral 

 impression of the faint light which the particles of air re- 

 ceive from all the other stars in the sky ; the intensity of the 

 luminous image and the size of the star thus appear increased. 

 In a much amended, and often contested, passage of Strabo, in 

 which there is a reference to looking through tubes, the en- 

 larged appearance of the stars, or heavenly bodies, is expressly 

 mentioned, although erroneously attributed to refraction.( 94 } 

 Light, from whatever source it may proceed, whether 

 from the sun, as solar light, or as reflected by the planets ; 

 from the fixed stars ; from rotten wood ; or as the product 

 of vital activity in glow-worms and other luminous animals, 

 always shows the same refractive properties. ( 95 ) But the 

 prismatic coloured images, or spectra, from different sources 

 of light, (from the sun and from the fixed stars), show a 

 difference in the position of the dark lines, which were first 

 discovered by Wollaston in 1808, and of which Fraunhofer 

 determined the position with great exactness twelve years 

 later. Fraunhofer had counted 600 dark lines (properly 

 speaking, interruptions, or defective parts, of the coloured 

 image or spectrum) : in the fine experiments of Sir David 

 Brewster with nitrous acid gas, in 1833, their number rose to 

 above 2000. It had been remarked that, at certain seasons 



