476 PHOtOGKAt^HY IN THE SERVICE OF ASTRONOMY. 



by a photo^niplier assisted by two aids. After <letailed estimate of 

 the outlay, Avhicli will result from this orgauizatioii, Mr. Gill thinks 

 that the budget of the central bureau ought to be fixed at a mini- 

 mum of 200,000 francs a year, but probably it will be necessary to put 

 it at 250,000 francs. The total outlay will thus rise to a little more 

 than 0,000,000 francs. This is the sum which appears necessary to se- 

 cure the publication of the catalogue of all the stars to the eleventh 

 magnitude, and that of the photographic re-production of all the stars 

 to the fourteenth magnitude, beside the cost of the telescopes, the sal- 

 aries of the astronomers, etc., expenses which we have already esti- 

 mated in the lump at 1,500,000 francs. It would be reasonable to sub- 

 tract from this the return from the sale of copies of the chart, which 

 will yield, alter Mr. Gill, about 1,000,000 francs in twenty-live years, or 

 enough to pay for the telescopes. 



Is this amount of 6,000,000 or 7,000,000 francs, to which we have thus 

 definitely arrived, exorbitant if we take into account the importance of 

 the results which we are concerned in obtaining ? It appears, on the 

 contrary, a trifling i)rice in comparison with what it would be necessary 

 to expend in order to arrive at the same results l)y the old processes. 

 In the present state of astronomy the formation of a catalogue com- 

 prising all the stars to the eleventh magnitude (which is the practical 

 limit of comparison stars in current observations, with the ordinary 

 observatory instruments) must be considered as an absolute necessity. 

 Now we know by experience, says Mr. Gill, that the cost of a single 

 exact meridian observation of a star (comprising the cost of reducliou 

 and publication) is never less than 10 francs, ami often surpasses this 

 figure. The catalogue which it is proposed to form with the aid of pho- 

 tograi)hy will comprise nearly two millions of stars, each of which will 

 have been determined two times in turn. To obtain the same number 

 of inde|)endent positions from meridian observations (supposing that 

 sufficiently powerful meridian instruments are found to furnish them) 

 it would be necessary evidently to spend about 50,000,000 francs. This 

 is eight times more than the cost of the photographic catalogue and 

 the general chart of the sky. In regard to the i)recision of the photo- 

 graphic positions, it will be superior to that of direct observations. It 

 snflices in this regard to mention the remarkable results which Mr. 

 Thiele, director of the Copenhagen observatory, has obtained by micro- 

 metric measures executed ui)on three plates of a st.ar cluster which had 

 been communicated to him by Messrs, Henry. 



It is necessary to say here a few words on the appearance which the 

 photographic images of stars present. These images, upon the ])lates, 

 have the form of small black disks, of a diameter nearly proportional 

 to the stellar magnitude, such as is figured upon celestial charts; their 

 dimensions increase gradually according as we prolong the exposure, 

 which it may be said in passing is a somewhat serious obstacle in 

 photometric researches, for the experiments of Mr. Scheiuer have shown 



