654 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



necessary to equal the accuracy obtainable with the optical parts of the 

 apparatus. 



The bars are rectangular, 2.3 cm. thick, with faces 3.6 cm. wide by 

 96.0 cm. long. The edges and corners are smoothly rounded to a radius 

 of something over one millimeter so that the plane silver films on the 

 faces do not end in edges which might determine spark discharge, but 

 round back on the sides of the bars. Solid electrical connection with 

 the films is made through two pairs of small intersecting holes drilled in 

 each bar on opposite sides near one end (Figure 2). A silver wire is 

 clinched in one hole of each pair and both holes are then filled with the 

 spongy gold used in dentistry firmly tamped in. When the bar is sil- 



Figure 3. Silvered glass bars wedged face to face. Side view and end view, 

 showing ring, wedges, and little quartz pieces separating bars. 



vered the film comes down perfectly solid and continuous over the glass 

 and the gold, and even if the film should break away from the wire 

 where the latter enters the bar, still through the other filled hole the 

 electrical connection would remain perfect. Reflection from the silver 

 film was always improved by careful dry polishing with settled rouge 

 on washed chamois. Minute scratches are produced, but the gain in 

 reflected light when the bars are in use close together, face to face, is 

 important. 



To hold the bars rigidly at a constant accurately known distance apart 

 small flat pieces of quartz are placed between them close to each edge 

 about every eight centimeters. These pieces of quartz were all cut 

 from the same plate which had been ground and polished at the Clarks' 

 more closely plane and uniform in thickness than a tenth of one per 

 cent of the total thickness which is 1.607 mm. Similar spacer pieces 



