56 ESSAYS AND OBSERVATIONS 



advanced one ftep beyond them. It is fur- 

 prizing, that, berore Sir Ifaac Ne'wton^ the 

 tvorld continued fo long entirely ignorant of 

 the true theory of light and colours ; and it 

 is no lefs (b, that, fince he quitted the fub- 

 iedt, no further difcovery of any moment has 

 yet been made amongft all the philofophical 

 Societies in Europe^ 



52. Many 



luftfe 5 finte they fend entire pencils of rays EKF, ELF to 

 the eye : and the vilible brightnefs of every point from L 

 towards M mufl decreafe gradually as from K to N : i. e. 

 The fpaces KN. LM will appear as dim (hadowy borders or 

 fringes adjacent to the edges of the opaque bodies. Wheii 

 the edge G is brought to touch the right line KF, the pe- 

 numbra's unite ; and, as foon a« it reaches NDF, the above 

 fhammenon begins : for it cannot pafs that right line without 

 Jneecing fome line «Di«' drawn from a point between N and 

 K, and, by intercepting all its rays that fell upon the pupil, 

 render it invifible. In advancing gradually to the line KDE, 

 it will meet other lines IDf, fD^, ire. and therefore rendef 

 the points h^ c, k^c. from N to K facceffively invifible • 

 and therefore the edge of the fixed opaque body CD mufl 

 feem to fwell outwards, and cover the whole fpace NK, 

 while GH by its motion covers MK. When GH is put to 

 a greater dillance from the eye, CD continuing fixed j the 

 fpace OP to be pafled over for intercepting NK is lefs j and 

 therefore, with an equal motion of GH, the apparent fwel- 

 ling of CD muft be quicker ; which is found true by expe- 

 rience If ML reprefents a luminous objefl, and REFQ^ 

 any plane expofcd to its light j the fpace FQ^will be entirely - 



ftaded 



