|f4 fHYSICAL OPTICS, 



iocedby the m- a fine needle. For greater convenience of obfervatlon, I placed 

 tcrferenceot ^ (maAl iookiiig glafs without the windou-Qiuttei, in i'uch a po- 

 hghc from both o ^ . r , , i • i- ^- i l • 



ed-cs. For in- "tion as to refled the fun's Ii^ht, rn a direction nearly honzon- 



terception on one (^I, upon theoppofite wa 1, and tocaufe the cone of divergmg 

 effeft. "S"^ ^^ P^'*^ ^^'*^'" ^ table, on which were leveral little Icreens ot 



card-paper. I brought Into the fun-beam a flip of card, about 

 one-thirtieth of an inch in breadth, and obferved its fliadow, 

 either on the wall, or on other cards held at different diftances. 

 Befides the fringes ol' colours on each fide of the ihadow, the 

 fliadow itfelf wasdivideJ by fimilar parallel fringes, of fmaller 

 dimenfions, differing in number, according to the diftance at 

 which the fliadow was obferved, but leaving the middle of the 

 Ihadow always white. Now thefe fringes were the joint eflfe6ls 

 of the portions of hght paflingon each fide of the flip of card, 

 and infleded, or rather diffracted, into the fliadow. For, a 

 little fcreen being placed a few inches from the card, fo as to 

 receive either edge of the fliadow on its margin, all the fringes 

 which had before been obferved in the fliadow on the wall 

 immediately difappeared, although the light infledted on the 

 other fide was allowed to retain its courfe, and although this 

 light muft have undergone any modification that the proximity 

 of the other edge of the flip of card might have been capable 

 of occafioning. When the interpofing fcreen was more re- 

 mote from the narrow card, it was neceffary to plunge it more 

 deeply into the fhadow, in order to extinguifli the parallel lines; 

 for here the light, diffracted from the edge of the objed, had 

 entered further into the fliadow, in its way towards the fringes. 

 Nor was it for want of a fufficient intenfity of light, that one 

 of the two portions was incapable of producing the fringes 

 alone ; for, when they were both uninterrupted, the lines ap- 

 peared, even if the intenfity was reduced to one-tenth or one- 

 iwentieth. 



(To be continued.) 



