( 633 ) 



the points were simiiltaiieoiisl)- ohjscrved tlir()iijj,h two magnifying 

 glasses. 



The position which tiie mirror M occnpies in this adjnstnient, 

 so tiie zero position, could be accurately determined by means of a 

 scale and a reading telescope with a cross wire. If now a difference 

 in pressure was brought about e.g. such that the mercury mirror 

 in the left leg fell somewhat and that ;n the right leg rose somewhat, 

 the adjustment of the two points on the mercury mirrors could be 

 again reached by turning the adjusting table, to which the manometer 

 is fastened, over a small angle to the left by means of the screw E, 

 for which at the same time the screw H was to be used, as the 

 bulbs BB are no perfect spheres, and have not perfectly the same 

 diameter either. 



When the glass points are again placed in the required way and 

 the reading glass is again read, it is clear, that 

 we can calculate the difference in level of the 

 mercury mirrors in a simple way from the 

 angle of rotation, Avhen we know the distance 

 I)etween the glass points. If this distance is /, 

 the angle of rotation a, the difference in height 

 between the two mercury mirrors is : 

 h ^zz I sin a. 



If we represent the deviation observed with the telescope ex})i'essed 



in m.m. a and the distance from the scale to the mirror 0, then 



a (I 



f(/ 2 a =z — , for Avhicli we mav write t(/ a = ~- pro^'ided a be very 



small. 



As further for small angles .s^/yz a = t</ a, we may write in this case : 



la 

 h :==! I t(l a =: — (1) 



If the angles are too large for this assumption, we get the following 

 form for h : 



la Z' 3 «" ^ 



' ~" 26 V ~^ Ï^V ^^^ 



The distance between the glass points was determined by means 

 of a comparator and amounted to 24.66 m.m. =: /. The distance 

 from the scale to the mirror amounted to 2735 m.m. = b. 



It now ])roved that when the deviation a was smaller than 100 

 m.m., formula (I) could be applied, whereas for larger deviations 

 formula (2) ai)peared to be required. 



The accuracy of adjustment, obtained in this arrangemejit with 



42* 



