542 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



It is to the latter that attention is particularly called in the present 

 note. 



In Figure 2 is shown the details of the device E. Each consists of a 

 plate of optically plain parallel glass. Opposite edges of this plate are 



polished and ruled with a dia- 

 mond midway between the upper 

 and lower surfaces. Each of 

 these side lines, as they may be 

 called, faces one of the large 

 glass scales. The position of 

 the scales is so adjusted that the 

 distance from plate to scale is 

 A as small as possible. On the 



under side of the plate E and 

 near its centre is a diamond line, 

 perpendicular to the sides turned 

 toward the scales. On the top 

 of the plate E rest two prisms 

 whose position and purpose will 

 be explained later. 



The position of the side lines 

 with reference to the glass scales 

 is read by means of cathetome- 

 ter telescopes not shown in the 

 drawing. 



The distance between each 

 ___^^^ side line and the lower surface 

 Vj of the plate can be determined 



with great accuracy before the 

 plate is mounted on the ap- 

 paratus, by observing under a 

 microscope tiie apparent dis- 

 placement of the line when 

 the opposite plain surfaces are 

 brought against a fixed stop. 

 Half this displacement is equal 

 to the distance from the line to the central plane of the plate. Know- 

 ing the thickness of the plate, tlie distance from the side lines to the under 

 surface of the plate is determined. 



Tills distance determined, we have left for determination the interval 



-Figure 1. 



