37 8 Dejcripthn cf 



circum (lance was particularly remarkable, and an obfervation 

 quite new. 



If a circle totally black, of about two lines in diameter, 

 be faflened on the middle of a large tlieet of paper or pafte- 

 board, and if this paper or pafleboard be placed in fueh a 

 manner as to be expofed fully to the light or the fun, if you 

 then approach it at the diftance of three or four feet, and 

 afterwards gradually recede from it, keeping your eve con- 

 llantly directed towards the black circle, it will appear always 

 to decreafc in fize the farther you retire from it, and at the 

 diftance of 33 or 34 feet will have the appear rice of a point. 

 If you continue ftill to recede, you will fee it again enlarge 

 itfclf; and it will feem to form a kind of cloud, the darknefs 

 of which deercafes more and more according as the circum- 

 ference becomes enlarged. The cloud will appear rtill to 

 increafe in fize, the farther you remove from it ; but at length 

 it will totally difappear. The moment of the difappearance, 

 however, cannot be accurate!) afcertained ; and the more 

 •jmetits were repeated, the more were the remits dif- 

 ferent. This is an obfervation perfectly accurate ; and hav- 

 ing myfelf made a ferries of experiments under like circum- 

 ftances, I am the more convinced of the truth of it. 



M. de SaulTure, having reflected for a long time on the _ 

 means of remedying this inconveniency, faw clearly, that, as 

 long as this cloud took place, no accuracy could be obtained; 

 and he discovered that it appeared in confequenec of the 

 contraft formed by the white parts which were at the grcateft 

 diftance from the black circle. He thence concluded, that if 

 the ground was left white near this circle, and the parts of 

 the pafteboard at the greateft diftance from it were covered 

 with a dark colour, the cloud would no longer be viiible, or at 

 lead almoit totally difappear. 



This conjecture was confirmed by experiment. M. dc 

 Sauflufe left ft white fpace around the black circle equal in 

 breadth to its diameter, by placing a circle of black paper 

 a line in diameter on the middle of a white circle three lines 



in 



