﻿tli e Micro- A zim om eier. 



387 



carrying the wire W, is capable o£ micro-metric adjustment 

 along a straight slide, whose line of motion is WS. By 

 these means the wire W may be brought with considerable 

 precision to the middle of the diffraction pattern, and the 

 reading of the micrometer noted. It is essential to sensk 

 tiveness that the width of the wire should be so chosen that, 

 in correct adjustment, the central bright band is almost 

 completely obliterated, only a faint "residual strip ,x of light 

 appearing on each side between the wire and the adjacent 

 dark ol the pattern. When this condition is fulfilled, a 

 displacement of the wire amounting to a very small fraction 

 of the width of the central baud will produce a perceptible 

 inequality in the brightness of the " residual strips." If 

 the wire employed is too thick, the sensitiveness will be 

 increased by placing a stop before the mirror, so as to reduce 

 the width of the reflecting surface, and thus spread out the 

 diffraction pattern to suit the diameter of the wire, 



4. When the illumination is monochromatic, and the slit 

 ideally narrow, the distribution of brightness in the diffrac- 

 tion pattern is that indicated in fig. 2. Abscissae, reckoned 

 from 0, represent distance measured horizontally in the 

 focal plane from the median line of the pattern, and ordi- 

 nates represent the corresponding brightness. AB is the 



Fi« 





H 





L 







' H\ 





if* 







. .____ : 3 





F ! ^^, . 





R 



A; E 



K 







G |B 

 M 



s 



width of the central bright band, and is equal to 2Xf/a t 

 where X is the wave-length of the light, / the focal-length of 

 the object-glass, and a the width of the mirror. The curve 

 of brightness touches the axis of abscissae at A and B, 

 corresponding to lines of absolute blackness in the pattern. 

 Now suppose that a vertical wire HK, LM is placed so as to 

 hide the central portion of the pattern, the axis of the wire 

 being in the focal plane, and its diameter only slightly loss 

 than AB. Thus the areas ADE, FGB will represent the 

 integral brightness (per unit of vertical length) oi' the 



2D2 



