26 



DISPLACEMENT INTERFEROMETRY BY 



ferometer must not only be parallel on entering the telescope T (fig. 9), but 

 they must be locally coincident at the mirror M; i. e., the two pencils from 

 the collimator L (M',N,N f being half-silvers) finally entering T, must very 

 nearly coincide. Otherwise, even when the path-difference is annulled 

 and there is perfect coincidence of the slit image, no fringes may be obtained. 

 This is often a great annoyance when the mirror M' is on a micrometer screw 

 (ri) normal to the face of the mirror; for on continuously displacing M' the 



:*' ^" 



-U 



rays cMT and dM'T separate more and more fully and the fringes soon 

 vanish, unless a fresh adjustment for local coincidence is made. It is for 

 this reason that the fringes are often so hard to find. The achromatics are 

 much less sensitive to this disadjustment than the spectrum fringes; but the 

 former are so mobile and easily lost that they have to be found as a rule by 

 the aid of the latter. 



To meet this difficulty there must be one mirror available which reflects 

 the component beam normally and which may be displaced parallel to itself ; 

 i. e., whose micrometer screw is parallel to the incident and reflected ray. 

 This condition is most easily secured by separating the auxiliary mirror into 

 the parts m and m', each normal to its respective ray, while m' only is on a 

 micrometer screw n 1 ' . Under these circumstances there is no difficulty in 

 finding the fringes after the adjustment for parallel rays and local coincidence 

 at M' has been made once for all by actuating the mirror m' in one direction 

 or another. Moreover, it makes no difference, within limits, how the paral- 



lelism and local coincidence are secured by moving any of the mirrors M,M', 

 N,N', m,m', all of which must be on three leveling screws. Finally, if the 

 mirror m and m' rotate as a rigid system about a common axis, it is still 

 possible to use mm' for the measurement of small angles. 



If the rays a and c, which may be of any length, are very long, the adjust- 

 ment shown in figure 10 is preferable, as the observer at T is near the 

 micrometer screw n'. Here M, N, N' are half-silvers. 



