EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 5 



direct attachment to the pier PP is at b and /. The ends of FF' are braced 

 by the rods FD and F'D in a vertical plane, and by rods at B (horizontal) 

 and C (oblique). The lower abutment of C is about a meter down toward 

 the rear, so that D, F', C, B is a. large tetrahedron. This arrangement is at 

 the same time adequately simple and firm, but it is not of course proof against 

 tremors. In fact, not a method was found by which these could be excluded 

 entirely. They exist in the pier. The optical parts are now attached to either 

 of the horizontal rods FF' by strong clamps of the usual type (reentrant 

 wedges). From the lamp at A, which may be either an arc or a Nernst fila- 



n 



FIGS. 3 and 4. 



ment, the light passes successively through the micrometer slit 5, the colli- 

 mating lens L, to the vertical plate of glass H on the horizontal pendulum. 

 Thence it is reflected to the opaque mirror N about 20 cm. behind the diagram, 

 and transmitted to the opposite mirror M. From both mirrors it is returned 

 to the plate of glass at H, after which the nearly coincident reflected and trans- 

 mitted beams pass to the left of the diagram to the far distant screen (2,000 

 cm.) on which they are caught and their distance apart, x, measured. 



The method best suited for visual observation, which alone is here at- 

 tempted, consisted in adjusting a clear glass millimeter scale (fig. 4) ss, about 

 15 cm. long, seen distinctly through the lens /, and noting the position of the 

 arriving beams of light, m and n, practically in focus on ss. A dark box open 

 at both ends B surrounds the beams and / is moved on a slide. 



The opaque mirrors (here plane) are necessarily adjustable around hori- 

 zontal and vertical axes and the micrometer slit must be very fine. At a is 

 a fine adjustment (horizontal and vertical axes) for the mirror M, though it 

 need only be used in the interferometry below. The gas-pipes, when partly 

 screwed, partly clamped, together, make a very serviceable framework for 

 experimental purposes. 



