PHOTOGRAPHY IN THE SERVICE OP ASTRONOMY. 475 



of London coutemplates establishing an observatory in New Zealand; 

 others, as those of Harvard College, Meudon, Poulkova, and Leyden, 

 will contribute actively in special researches for the advancement of the 

 coninion work. These relate to the following: The preparation of reti- 

 cules whose image impressed on the plate can furnish the standard for 

 micrometrical measures, and i)ermit to recognize deformations of the 

 sensitive film ; it is necessary to make a preliminary study of the scale 

 of photographic magnitudes of stars ; to consider the means of determin- 

 ing the optical distortion of the field of the telescope; to study the 

 method of measuring and reducing the plates, etc. It is of much im- 

 portance to thus clear up the ground before commencing the execution 

 of the chart, in order not to be retarded afterwards by nuforeseeu ob- 

 stacles. We can now attempt to gain an idea of the expense Avhich the 

 projected enterj^rise will involve. The cost of the construction of a 

 l)hotographic refracting telescope is estimated at from 50,000 to 00,000 

 francs; the fifteen or sixteen telescopes which will be needed will cost, 

 then, nearly 1,000,000 francs. 



In adding to this sum the price of the plates, and for each observa- 

 tory, the appointment of at least two operators during two years, we 

 arrive at a total in the neighborhood of 1,500,000 francs. It is true that 

 the instruments remain the property of the establishments which or- 

 dered their construction and that the work could be confined to the ex- 

 isting personnel. But the execution of the photographs is not the 

 most costl}' part of the enterprise. David Gill, in a memoir inserted 

 in the first fascicule of the Bulletin of the International Permanent Com- 

 mittee, has elaborated a detailed i>lan of organization of the office work 

 which ought to be accomplished in view of the publication of the results, 

 and which will consist, before all, in the measure and reduction of the 

 plates designed for the formation of catalogue.^ This work, of a nature 

 so special, says Mr. Gill, requires so ]>erfect an experience and so skill- 

 ful an organization, in order to be conducted to a successful termination 

 without too much cost, that it will be most necessarily in charge of a 

 central bureau. Under these conditions this will be an outlay to be 

 provided for. 



Mr. Gdl supposes that the plates of the catalogue will be made, as 

 those of the chart, in duplicate, and that each plate will cover four de- 

 grees square, so that the total number of plates will amount to about 

 twenty thousand. The labor of measuring and reducing must be done 

 under the direction of an energetic and skillful chief, hy young persons 

 of both sexes of average intelligence; not less than thirty will be 

 needed, and the entire completion of the work will demand from seven- 

 teen to twenty-five years. The publi(;ati()n of the catalogue will keep 

 up with the calculations. For the chart of the sky, so called, it will 

 suffice to issue to subscribers, namely, to observatories, societies, or na- 

 tions wiio have relations with tiie bureau, i)ositives on glass, obtained 

 by means of the original negatives. These copies Vrill be executed 



