CHAPTER IV. 



THE DISTANCE BETWEEN TWO PARALLEL TRANSPARENT PLATES. 



29. Introductory. The problem of finding the distance separating two 

 parallel glass disks, as well as their degree of parallelism, is frequently one of 

 practical importance. Thus, in my work on the repulsion of two such disks, 

 it would enter fundamentally, and it has long been my intention to repeat 

 that work with two half-silvered glass disks, for comparison with the case of 

 metallic disks. It has since occurred to me that the method devised by my 

 son, Mr. Maxwell Barus, and myself* would probably be ideal for the purpose, 

 both for very small distances (within o.i mm.) as well as for distances ten or 

 more times larger. This method admits of use of the film grating, and there 

 are three types of interferences of successive orders of fineness, the first virtu- 

 ally involving the colors of thin plates (resolved spectroscopically), the other 

 two being dependent on diffraction. To measure the thickness of the air-space 

 it would be necessary to count the number of fringes between two definite 

 Fraunhofer lines only, supposing the constants of the grating to be given. 



30. Apparatus. The apparatus has been designed for transmitted light, 

 in preference, though the case of reflection is also available. 



A 



^r 



cJ 



Jl 



^ 



J^ 



c/M. 



5Q 



5! 



MM, figure 50, is the base of a Fraunhofer micrometer, firmly attached 

 below to a massive tripod (not shown) . 55 is its raised slide, and E the head 

 of the micrometer screw, reading to icr 4 cm. The open case A is screwed to 

 the slide 55 and contains the glass plate H half-silvered on the right. H is 

 attached to a plate of brass, on the plane-dot-slot principle, and may there- 

 fore be rotated around the vertical and horizontal axis by aid of a rearward 

 spring mechanism (not shown) and the adjustment screws a, b, b' (the last 

 not visible). The grating G, with a ruled face on the left, is similarly carried 

 by the open rectangular case B, screwed down to the base M of the micrometer. 

 Thus B is stationary, while A moves. Three adjustment screws, c, d, d' (d f 

 not shown), and a spring pulling to the right suffice to rotate G around the 



* C. Barus and M. Barus, Carnegie Inst. Wash. Pub., No. 149, Part I, Chapters II and 

 III. 1911. 



69 



