762 Prof. H. Rubens and Mr. H. Hollnagel on 



meter is shown in fig. 1 ; A-A is a small dividing engine 

 upon whose prismatic way B-B a brass carrier C was fixed ; 



JFisr. 1. 



B 





-K 



MA 



to allow the passage of the beam of rays the latter was 

 turned out to a diameter of 5*5 cm. ; it supported a brass 

 ring D which carried one of the above mentioned quartz 

 plates G'. By means of the three adjusting screws E and 

 the three guiding pins F, the adjustment of this plate was 

 facilitated. The second quartz plate Gr was held firmly in 

 another ring H, attached to the movable slide I of the 

 dividing engine. We made use of two sets of quartz plates, 

 a plane-parallel and a wedge-shaped pair ; the former were 

 of a thickness of 0*6 mm., while the latter tapered from 

 0*8 mm. to 0'4 mm. As the mean pitch of the thread of 

 the dividing engine was 0*5228 mm. and the micrometer 

 drum K was divided into one hundred parts, a rotation of 

 one division of the drum corresponded to a displacement o£ 

 5'23/a of the slide. 



To demonstrate that the slide moved accurately enough 

 for the present purpose the following experiment was carried 

 out : — After adjusting the quartz plates as nearly parallel 

 as possible by means of the screws, E, they were brought 

 into contact by turning the drum, K. If now they were 

 illuminated by sodium light a series of irregular, con- 

 centric, interference rings or bands appeared by transmitted 

 as well as by reflected light. These are the well known 

 Newton's rings, caused in the air-layer by a slight warping 

 of the thin quartz plates. In that portion of the inter- 

 ference field utilized in the measurements, i. e. aboutl2 sq. cm, 

 there were generally not more than four or five of such 

 rings. If now the quartz plates were gradually separated 

 by a slow rotation of the micrometer drum, these rings 

 moved rapidly toward the centre of the fringe system, 



