2 Mr. A. A. Michelson on the Application of 



bands from two pencils diverging from the same source (width 

 a) at an angle /3, it is necessary that either ft or a be very 

 small. 



Fig. 1. 



Thus, in fig. 1, let us take 



a= ee x = width of the source. 



d= distance of source from the objective. 



b = SSx = distance between the slits. 



Also put $!? = &,-= a, y =/3. 



Then the usual statement is that the interference-fringes 

 vanish when S^ — ^>e=^fta = ^bu = ^\, or when 



X 



But j is the " limit of resolution " of the telescope of 

 aperture b, and if this be denoted by a we have 



Or, in words, the fringes disappear when the source subtends 

 an angle which can just be resolved by the telescope. 



The experiment was first tried with an objective of 45 

 millim. effective diameter (distance between the slits) at a dis- 

 tance of ten metres from an adjustable slit which served as 

 the source. 



It was found that the first indication of indistinctness 

 occurred when a was 0*08 millim. wide, and at 0*14 millim. 

 the fringes almost vanished. 



Bid on continuing to widen the slit they again became clearly 

 visible, to disappear and reappear at regular intervals. 



Now, though it might with truth be urged that the ob- 

 servation of the indefinite vanishing of interference-fringes 

 depends so much on the attendant circumstances, and especially 

 on the condition of the observer, that it can scarcely be called 

 a precise measurement, yet the statement applies no longer 



