3/. Dc Sau [fare's DiafJja?W7?ietcr. 379 



in diameter, fo that the black circle was only furrounded by 

 a white ring a line in breadth. The whole was patted up- 

 on a oreen ground. A green colour was chofen, becaufe it 

 was dark enough to make the cloud difappear, and the 

 eafieft to be procured. 



The black circle, furrounded in this manner with white 

 on a green ground, difappeared at a much lefs diftanee, than 

 when it was on a white ground of a large fize. 



If a perfectly black circle, a line in diameter, be paftcd on 

 the middle of a white ground expofed to the open light, I can 

 obferve it at the diftanee of from 44 to 45 feet; but if this 

 circle be furrounded by a white ring a line in breadth, while 

 the reft of the ground is green, I lofe fight of it at the dif- 

 tanee of only 15 \ feet. 



According to thefe principles M. de Saufiure delineated 

 feveral black circles, the diameters of which increafed in a 

 geometrical progreffion, the exponent of which was \. U is 

 Imallefi circle was | or era of a line in diameter; the fecond, 

 0-3 ; the third, 0-45 ; and fo 011 to the fixteenth, which was 

 87*527, or about 7 inches p.l- lines. Each of thefe circles 

 'mounded by a white ring, the breadth of which was 

 equal to the diameter of the circle, and the whole was pafted 

 on a green ground. 



M. de Saufiure, for his experiments, felecled a ftraight 

 road or plain of about 12 or 1500 feet in circumference, 

 which towards the North was bounded by trees or an afcent. 

 Thole who repeat them, however, mull pay attention to the 

 following remarks: — When a perfon retires backwards, keep- 

 ing his eye conftantly fixed on the pafteboard, the eye be- 

 comes fatigued, and foon ccafes to perceive the circle; as foon 

 therefore as it ccafes to be difiinguifhable, you mull fuffer 

 your eyes to reft; not, however, by fiiutting them, for thev 

 would when again opened be dazzled by the light, but by 

 turning them gradually to loine lefs illuminated object in the 

 horizon. When you have done this for about half a minute. 



and 



