PORTION OF THE COSMOS. VISUAL POWER. 51 



be equal to the thickness of the rod. A line is seen much 

 further than a point, even if the breadths of the two are 

 equal. Arago examined the influence of form in this 

 respect by angular measurements of the lightning conductors 

 visible from the Paris Observatory. Determinations of the 

 smallest optical angle of vision under which terrestrial 

 objects can be recognised by the naked eye have always 

 continued to advance progressively to smaller and smaller 

 quantities ; from Robert Hook, who declared a full minute 

 to be absolutely necessary, to Tobias Mayer, who required 

 34" for a black spot on white paper, and further to Leuwen- 

 hoek's spiders' threads, which can be seen by persons of 

 very ordinary powers of vision under an angle of 4"' 7. In 

 the most recent and very exact experiments of Hueck on 

 the motion of the crystalline lens, it was barely possible to 

 see white lines on a black ground at an angle of 1"% a 

 spider's thread at 0"-6, and a fine shining wire at 0"'2. 

 The problem does not admit of a strict numerical solution, 

 as the result depends on the shape of the objects, their 

 illumination, their contrast with the back ground from 

 which they detacli themselves, and the movement or still- 

 ness, as well as the nature, of the intervening atmospheric 

 strata. 



I was much impressed by a circumstance which occurred 

 during my stay at a beautiful country-seat belonging to the 

 Marques de Selvalegre, at Chillo (not far from Quito), from 

 whence the long extended ridge of the Volcano of Pichincha 

 was seen at a horizontal distance, trigonometrically measured, 

 of 85000 Paris (90590 Eng.) feet. My fellow-traveller 

 Bonpland, who was then engaged alone on an expedition to 

 the Volcano, was recognised by the Indians standing near 

 me as a white point moving alon^ the face of a black ba- 



