DIOPTRIC TUBES. 53 



nicus should have had so great a knowledge of the true 

 system of the universe ; and that Tycho Brahe should have 

 been so familiar with the methods of practical astronomy 

 before the discovery of the telescope. Long tubes, 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 observations 

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

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

 mities 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 dis- 

 covered 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 conceals a great portion of the 

 disturbing light (rayons perturbateurs) diffused in the atmo- 

 spheric 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 frequently emendated and much contested passage of 

 Strabo, in which mention is made of looking through tubes, 

 this " enlarged form of the stars" is expressly mentioned, and 

 is erroneously ascribed to refraction. 5 



5 The passage in which Strabo (lib. iii. 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 rising as well as at its setting, 

 because at these times a larger mass of exhalations rises from 

 the humid element ; and the eye, looking through these exha- 

 lations, sees images refracted into larger forms, as observed 

 through tubes. The same thing happens when the setting 



