24 DISPLACEMENT INTERFEROMETRY BY 



45 and the vertical) they may be erected by rotating either compensator 

 or both around a horizontal axis and enlarged by rotating around a vertical 

 axis. The two compensators should be actuated together in opposite direc- 

 tions if there is danger of the fringes leaving the field; i. e., if the adjust- 

 ment is considerable. Similarly, if the fringes are more nearly horizontal, 

 and particularly when horizontal fringes are wanted, the fringes may be 

 leveled by rotating the compensators in opposite directions around a vertical 

 axis and enlarged by rotating around a horizontal axis. Horizontal fringes, 

 which climb up and down the broad white slit-image and may be made 

 quite large, are often very advantageous. The illuminated field is much 

 more extensive in the vertical direction. 



For slight adjustments it is convenient to have the compensators nearest 

 at hand rotate in the same direction as the fringes. This may be done by 

 working either on one side or the other of the center of fringes. The com- 

 pensators may easily be ma- 

 nipulated by hand (without ,-' T~ "\ 

 tangent screws) and they are 

 most efficient when nearly 

 normal to the respective 

 beams. To pass from vertical 

 to horizontal fringes one would 

 first rotate the compensators in opposite directions around a horizontal axis 

 until the fringes are inclined about 45, after which the further rotations 

 would be made in opposite directions around a vertical axis. The motion of 

 fringes indicates the proper direction of the rotation of the plates. 



To account for these apparently complicated effects, it is sufficient to re- 

 call that the compensators displace the center of fringes, usually enormously 

 distant outside of the field of view, and besides that invisible with white light ; 

 for fringes are visible only in the narrow strip for which the spectrum fringes 

 are very large and centered. Hence the result of rotation around a hori- 

 zontal axis is to change fringes (fig. 8) of the type a, through c (vertical) 

 into b, while the center moves downward, and vice versa. Again, rotation 

 around a vertical axis changes fringes of the type a, through c' (horizontal) 

 into d, as the center moves from left to right, and vice versa. The effects are 

 necessarily opposite for the two beams. If the fringes are made vertical 

 as at c, rotation of a compensator around the vertical axis can have no effect 

 of rotation of fringes ; for the center moves in the line of symmetry ; but the 

 effective or differential thickness of plates (e) is changed and hence the 

 fringes are increased or decreased in size. 



In view of the presence of compensators, C and C', figure 7, the original ad- 

 justment is much simplified ; for it is necessary merely that the spots of sun- 

 light on the mirrors at a and b, figure 7, and at a' and b r , one or more meters 

 off, be at the same level and at the same distance apart, nearly. The accu- 

 rate adjustment at c and d for coincidence horizontal and vertical is then 

 made with the telescope at T. When the distances are approximately equal, 



