DIOPTRIC TUBES. 43 



which were certainly employed by Arabian astronomers, and 

 very probably also by the Greeks and Romans, may indeed, 

 in some degree, have increased the exactness of the observa- 

 tions by causing the object to be seen through diopters or slits. 

 Abul-Hassan speaks very distinctly of tubes, to the extremi- 

 ties of which ocular and object diopters were attached ; and 

 instruments so constructed Were used in the observatory 

 founded by Hulagu at Meragha. If stars be more easily 

 discovered during twilight by means of tubes, and if a star 

 be sooner revealed to the naked eye through a tube than 

 without it, the reason lies, as Arago has already observed, in 

 the circumstance that the tube eonceals a great portion of the 

 disturbing light {rayons perturbateurs) difiused in the atmos- 

 pheric strata between the star and the eye applied to the tube. 

 In like manner, the tube prevents the lateral impression of the 

 faint light which the particles of air receive at night from all 

 the other stars in the firmament. The intensity of the image 

 and the size of the star are apparently augmented. In a fre- 

 quently emendated and much contested passage of Strabo, in 

 which mention is made of looking" through tubes, this " en- 

 larged form of the stars" is expressly mentioned, and is erro- 

 neously ascribed to refraction.* 



* The passage in which Strabo (lib. hi., p. 138, Casaub.) attempts to 

 refute the views of Posidonius is given as follows, according to the 

 manuscripts : " The image of the sun is enlarged on the seas at its ris- 

 ing as well as at its setting, because at these times a larger mass of ex- 

 halations rises from the humid element ; and the eye, looking through 

 these exhalations, sees images refracted into larger forms, as observed 

 through tubes. The same thing happens when the setting sun or moon 

 is seen through a dry and thin cloud, when those bodies likewise appear 

 reddish." This passage has recently been pronounced corrupt (see 

 Kramer, in Strabonis Geogr., 1844, vol. i.,p. 211), and 61 vilXuv (through 

 glass spheres) substituted for 6i av"kuv (Schneider, Eclog. Phys., vol. ii., 

 p. 273). The magnifying power of hollow glass spheres, tilled with 

 water (Seneca, i., 6), was, indeed, as familiar to the ancients as the ac- 

 tion of burning-glasses or crystals (Aristoph., Nub., v. 765), and that of 

 Nero's emerald (Plin., xxxvii., 5) ; but these spheres most assuredly 

 could not have been employed as astronomical measuring instruments. 

 (Compare Cosmos, vol. ii., p. 245, and note \.) Solar altitudes, taken 

 through thin, light clouds, or through volcanic vapors, exhibit no trace 

 of the influence of refraction. (Humboldt, Recueil d'Observ. Astr., vol. 

 i., p. 123.) Colonel Baeyer observed no angular deviation in the heli- 

 otrope light on the passage of streaks of mist, or even from artificially 

 developed vapors, and therefore fully confirms Arago's experiments. 

 Peters, at Pulkowa, in no case found a difference of 0"-017 on compar- 

 ing groups of stellar altitudes, measured in a clear sky, and through 

 light clouds. See his Recherches sur la Parallaxe des Etoiles, 1848, p. 

 80, 140-143 ; also Struve's Eludes Stellaires, p. 98. On the application 

 of tubes for astronomical observation in Arabian instruments, see Jour- 



