A. MATHEMATICS AND ASTRONOMY. 53 



a naval party, in which the corresponding officers were of- 

 ficers of the navy. The Coast Survey furnished the astron- 

 omers for two parties, and the Observatory furnished the 

 chiefs for two more. The two remaining chiefs were dis- 

 tinguished civilian astronomers, each the director of an ob- 

 servatory. 



The plan of observation adopted by the commission was 

 in many respects different from that used elsewhere, be- 

 ing worked out quite independently. Its distinguishing 

 features were that the observations should be made on the 

 same plan at all the stations, that the main reliance should 

 be placed on photographs of the phenomena, and that these 

 photographs should be taken on a peculiar plan. The essen- 

 tial feature of the plan is that the image of the sun is thrown 

 into the photographic dark room by being reflected from a 

 flat mirror, the rays after reflection passing through a lens 

 of forty feet focal length. This lens is so adjusted that the 

 image is formed on the photographic plate, which is firmly 

 mounted on an iron pier in the dark room. This plan is ex- 

 tremely convenient in photographing, and one of the best 

 illustrations of its advantages is found in the fact that at not 

 a single one of the stations was there any mishap which in- 

 terfered with the taking of the photographs during the crit- 

 ical moments of the transit. Another circumstance worth 

 noting is that the entire apparatus was of American manu- 

 facture, the most important part being made by Alvan Clark & 

 Sons. The success of these ingenious artists with the mir- 

 rors of the photographic apparatus was especially gratifying, 

 as it was absolutely necessary to the success of the plan that 

 these should be made with a degree of perfection which Eu- 

 ropean astronomers feared might be unattainable. 



As stated in our previous volume, the Americans occupied 

 three northern and five southern stations, an unequal divis- 

 ion between the two hemispheres being made because the 

 chance of good weather was much greater in the northern 

 hemisphere than in the southern, and it was desirable to have 

 as nearly as possible an equal number of observations on the 

 two sides of the equator. The northern parties, w T ith their 

 material, were sent from San Francisco to Nagasaki by the 

 Pacific Mail steamships, whence two of them were distrib- 

 uted to their stations by naval ships, one remaining in Na- 



