70 



eye. Adams very correctly observed that a long and slen- 

 der staff can be seen at a much greater distance than a 

 square whose sides are equal to the diameter of the staff. 

 A stripe may be distinguished at a greater distance than a 

 spot, even when both are of the same diameter. Arago has 

 made numerous calculations on the influence of form (outline 

 of the object) by means of angular measurement of distant 

 lightning conductors visible from the Paris Observatory. The 

 minimum optical visual angle at which terrestrial objects can 

 be recognized by the naked eye has been gradually estimated 

 lower and lower from the time when Robert Hooke fixed it 

 exactly at a full minute, and Tobias Mayer required 34" to 

 perceive a black speck on white paper, to the period of Leeu- 

 wenhoek's experiments with spider's threads, which are visible 

 to ordinary sight at an angle of 4"* 7. In the recent and 

 most accurate experiments of Hueck, on the problem of the 

 movement of the crystalline lens, white lines on a black 

 ground were seen at an angle of 1"'2 ; a spider's thread at 

 0"'6 ; and a iine glistening wire at scarcely 0"-2. This pro- 

 blem does not admit generally of a numerical solution, since it 

 entirely depends on the form of the objects, their illumination, 

 their contrast with the back-ground, and on the motion or 

 rest, and the nature of the atmospheric strata in which the 

 observer is placed. 



During my visit at a charming country-seat belonging to 

 the Marques de Selvalegre, at Chillo, not far from Quito, 

 where the long extended crests of the volcano of Pichincha 

 lay stretched before me at a horizontal distance, trigonometri- 

 cally determined at more than 90000 feet, I was much 

 struck by the circumstance that the Indians who were 

 standing near me distinguished the figure of my travelling 

 companion Bonpland (who was engaged in an expedition to 

 he volcano) as a white point moving on the black basaltic sides 

 of the rock, sooner than we could discover him with our teles- 



