SCHLESINGER— NEW PHOTOGRAPHIC INSTRUMENT. 485 



another group of equatorial stars, and the plate is then taken out of 

 the camera and developed. It is clear that the two sets of chrono- 

 graphic records, together with the measurement in right ascension 

 of the two exposures, will give us the right ascension of one group 

 if that of the other is known; similarly for the declinations, except 

 of course that the clock is not involved. 



The method is liable to several sources of error: (i) accidental 

 errors in the measurement of the plates and of the chronographic 

 records. (2) Errors in the assumed rate of the clock. (3) Errors 

 due to the movement of the pier in the interval between the two 

 exposures. It is certain that the first of these is smaller than in the 

 best work that is possible by visual methods, and in addition we are 

 freed from personal equation in all its forms. This observatory 

 possesses an excellent Riefler clock whose rate for ten hours (the 

 longest interval between exposures that it is feasible to employ), 

 we should be able to determine with a probable error not exceeding 

 0.005 second of time. Several years ago we set up a stationary 

 camera upon a pier pointed at the polar regions and secured expos- 

 ures every few minutes on a number of stars. The measurement and 

 preliminary discussion of these plates proved that the pier is liable to 

 very small movements during the course of a single night, and the 

 error from this source is not greatly to be feared. 



We have constructed such an instrument as this in the observa- 

 tory shops from such material as we happened to have at hand. A 

 trial of it has encouraged us to reconstruct it in more permanent 

 form, and in particular we are having made for it an accurate driv- 

 ing sector and worm. It is proposed to put the method and instru- 

 ment to a very severe test by extending the observations through an 

 entire year, coming back to the group that forms our starting point 

 by means of six steps. 



The camera is being tried in the equator only because this simpli- 

 fies the construction. A slight and obvious modification will make 

 it applicable to any decHnation whatever. In this more general form 

 the device, if successful, will enable us to ascertain the right ascen- 

 sion and the declination of stars in any portion of the sky providing 

 that we know beforehand the positions of any other stars in about 



