Y. — On the Permanence of the Rutherfurd Photographic 



Plates. 



BY HAROLD JACOBY. 



Read April 6th, 1896. 



1. It has been evident for some time that the great mass of 

 photographic plates which have been accumulating in the hands 

 of astronomers can not be subjected to measurement without 

 considerable delay. Indeed, this delay will probably be as great 

 as a quarter of a century in the case of some of the plates which 

 will ultimately be measured with precision. In other cases, no 

 doubt, the measurement will never be made. It is clear that very 

 great interest attaches to the question of how long the plates can 

 be kept without impairing the degree of precision attainable in 

 their measurement. In the case of the Rutherfurd plates this 

 question is especially important, because the major portion of 

 the whole collection of plates is still unmeasured. If these plates 

 admit of accurate measurement they furnish us with a source of 

 information of the greatest value, since they are now about 

 a quarter of a century old, and will therefore supply a unique 

 contribution toward the study of inter-stellar motion. 



It is fortunately possible to decide this question in a very sim- 

 ple manner, as far as the Rutherfurd plates are concerned. It is 

 merely necessary to remeasure some of the plates which were 

 very carefully measured under Mr. Rutherfurd's direction soon 

 after thej'^ were made. A simple comparison of the new measures 

 with the old ought then to bring to light any possible changes in 

 the plates. It is true that such a comparison will not furnish 

 definitive information as to the permanence of the modern gela- 

 tine dry-plates, such as are now everywhere in use for astronom- 

 ical photography. For the Rutherfurd photographs were made 

 by the old wet-plate process, using albumenized plates. Yet it 

 is not improbable that the modern dry-plates will equal in dura- 

 bility^ the ones used by Rutherfurd, so that the very favorable re- 

 sult obtained in the present paper is most encouraging to the 

 hope that the measurement of the plates now being made will be 

 possible at epochs quite remote from the present time. 



