( 216 ) 



Of the greatest importance is the measurement of the period T 

 which in formula (7) occurs squared. The total amount of T is 

 always very small, in some cases even less than 1 mm. so that it 

 is desirable to carry out this measurement with an accuracy of a 

 very small fraction of a millimeter. 



For this purpose I first used an excellent astronomical instrument, 

 destined for the investigation of celestial photograms, and which 

 was put at my disposal by the kindness of the director of the Leyden 

 Observatory ^). 



Later I used exclusively an ordinary microscope stand, pattern 

 I« of the firm Carl Zeiss. The stand is provided with the large 

 movable cross table of the same firm. On this table a small wooden 

 board is fastened, which follows the cross movements, being always 

 supported by a smooth plane on which it can slide easily. On this 

 board, which has in the middle a spacious opening, the photographic 

 plate to be measured is placed. 



As object-glass a^ is used with a focal distance of 40 mm., while 

 for the eye-piece that of Ramsden is used, also made by Zeiss, with 

 micrometer screw and drum reading. The eye-piece is so adjusted 

 that the direction of motion of the measuring wire coincides with 

 the direction of the abscissae of the photogram. After the microscope 

 has been sharply focussed on the photogram and at the same time 

 care has been taken that the image to be measured and the image 

 of the crosswire lie in the same optical plane, the microscope tubes 

 are firmly screwed on to the stand by means of a clamp, which is 

 expressly made for this purpose. The tubes must be quite immovably 

 connected with the stand. Also the eye-piece is screwed on to the 

 microscope tube so that it can be touched with the hand without 

 observing the slightest displacement of the crosswire with respect to 

 the image to be measured. For measurements requiring some accuracy 

 these arrangements are indispensable. 



With the microscope tubes pushed in, 1 scale division of the drum 

 corresponds to a little less than 5 ft on the photogram, and since 0.1 

 of a scale division can easily be estimated, the error in the reading 

 is less than 0.5 [i. It would be a mistake, however, to suppose that 

 also the result of the whole measurement can now be known with 

 the same accuracy. For there remain errors of a different origin. 



So e. g. we start in our measurements from the shadow lines 

 which in the photogram form the net of square millimeters, a base 



1) I wish to express here my best thanks to Prof, van de Sande Bakhuyzen 

 for his kind assistance. 



