( 507 ) 



order to render the raoimting more simple, the coelostat mirror would 

 be used for both slit-spectrographs instead of the southern siderostat 

 niiiTor. 



The Eclipse Committee had consented to an alteration of the 

 clock-work of the coelosiderostat, so that the number of the wheels 

 in outside gearing, of which some were very difficult to get at for 

 cleaning, was reduced from 5 to 2 ; I had this constructed by 

 Mr. Gautier. The clock-work was received here in the middle of 

 July 1905. It has worked excellently. 



The two spectrographs were carefully examined and cleaned. 



I determined the zeros of the micrometer screws, indicating the 

 slit-width, by means of diffraction observations, a method which 

 allows of an accuracy of some microns. The adjustment of the slit 

 in the principal focus of the collimator object glass, which could not 

 be easily done with the desired accuracy in a direct way, was 

 indirectly performed in the following manner. By photographs made 

 according to the method of Hartmann I determined the position of 

 the photographic plate in the principal focus of the camera object 

 glass. Then the collimator was placed as a source of light in front 

 of the camera object glass, and the same photographs were made 

 again. From the difference between the focus found now and the 

 principal focus found before we could derive how much the slit 

 had to be removed in order to bring it in the principal focus of 

 the collimator object glass. 



In this experiment it appeared however that both the collimator 

 and the camera object glass of the large spectrograph had a very 

 great spherical aberration, and with full aperture they were unfit to 

 form sharp images, while a diaphragm would cause a loss of light 

 which, with a view to our purpose, was inadmissible. 



Therefore I ordered of Steinheil new object glasses; a single 

 object glass with a field of sharp definition of 2° for the collimator 

 and a compound one with a field of sharp definition of 15° for 

 the camera. 



Neither of them were in store and in the available time this firm 

 could only supply object glasses of the first kind of which two pieces 

 were sent to me. 



Although their fields of sharp definition were too small for the 

 camera I determined to try them also for this purpose, as the 

 middle of the spectrum was of chief interest for the photograph 

 intended. The spherical aberration was exceedingly small. 



Meanwhile the Steamboat Company had sent word that every- 

 tiiing had to be shipped 10 days earlier than had been agreed upon, 



35 



Proceedings Royal Acad. Amsterdam. Vol. VIII. 



