PROGRESS IN ASTRONOMY PUISEIJX. 141 



That photography can deal with small intervals with rapidity and 

 great accuracy has been repeatedly shown. But it is desired to 

 free the photographic plate from any dependency upon the merid- 

 ian circle. 11. H. Turner' has devised a very complete scheme. 

 Pie proposes to" gather, on the same plate, images of very distant 

 portions of the sky, and believes he can register with the necessary 

 precision the beginning of each exposure. The plan of Turner in- 

 cludes the use of two photographic telescopes mounted at right 

 angles to each other in the equator and adjusted with a prismatic 

 mirror. The project has received the approbation of the Astronom- 

 ical and Astrophysical Society of America.^ 



But these methods have not received such emphatic approval 

 everywhere. Before the Royal Astronomical Society Sir David Gill, 

 Sir William Christie, and A. E. Conrady showed numerous reasons 

 for fearing errors in the use of the new methods. According to them, 

 the status of the meridian circle in fundamental astronomy is not 

 yet in any way undermined. 



In the past rival processes, even when recommended by illustrious 

 names, have not realized the hopes of their promotore. Such was the 

 case with the zenith telescope, or the alt-azimuth as introduced at 

 Greenwich by Airy and the floating telescope in the hands of Chandler, 

 Sampson, and Bryan Cookson. But it is true that photography 

 introduces a new element into the problem and the experiments now 

 in progress at Oxford deserve attention. 



The resources of the photographic method will be 3'et greater when 

 it is possible to utilize a greater field upon a single plate without the 

 deformation of the images near the edges. Theoretically, curved 

 plates could be employed which would comport better with the focal 

 surfaces of the objectives. Such an attempt was made some 20 years 

 ago at the beginning of the Celestial Chart project. It was not con- 

 tinued in use because such curved plates were not adapted to the 

 micrometrical measuring machines. The difficulty has been over- 

 come as the result of recent experiments at the Harvard College 

 Observatory. The sensitive plate serves as the cover of a metallic 

 box, from which the air may be removed. The atmospheric pressure 

 ui)on the ])late produces the desired curvature. When the air has 

 been reintroduced, after the exposure, the plate losses its curvature 

 and is developed and measured -vN-ithout difficulty. 



A conference was held in Paris in October, 1911, by the representa- 

 tives of all the great nations which publish official ephemerides 

 (France, Germany, the United Kingdom, the United States of Amer- 

 ica). The resolutions unanimously adopted after very amicable dis- 

 cussions will introduce important economies in efforts, which up to 



> Monthly Notices, vol. 71, pp. 422, 427. 2 The Observatory, vol. 34, p. 2;J3. 



