324 Prof. F. L. 0. Wadsworth on the Resolving Power of 

 In this case <t = «, and we have therefore 



tan— = oo . 



or 



7 = l*5a ; 2'5a, 3-5a, &c, 



or the minima occur at points \a further from the centre than 

 when the source is a line of negligible width *. 



&o 



The integral was evaluated in the same manner for different 

 values of a both smaller and larger up to cr = 3«. The diffrac- 

 tion-patterns of two sources of width cr = « and o = 3a are 

 shown in fig. 2, in dotted lines. 



In order that a double line may be resolved it is necessary 



* Since the position of the minima in this case depends on the angular 

 magnitude of the source <r as well as on the aperture of the telescope, 

 it follows that by covering the objective of the latter by a rectangular 

 opening of known size, and then measuring by means of a micrometer 

 the positions of the minima of the diffraction-pattern, the value of <r 

 may be determined from the above equations. Experiments on a large 

 number of slits of varying width and holes of varying diameters (for 

 which the positions of the minima are slightly different) showed that 

 when the source was sufficiently bright to give well-marked minima, 

 single observations gave results which were at least five times as accu- 

 rate as could be obtained by direct micrometric measurement of the 

 image with full aperture of the telescope. This method is, however, con- 

 siderably less accurate than the refractometer method of Professor 

 Michelson which is fully described in the earlier part of the paper 

 referred to, and the observations are therefore not given at length. 



