ASTRONOMY 395 



of curvature when exposed to tlie sun's rays. As that question is of 

 vital importance in the theory of the horizontal photo-heliograph, it 

 was thought desirable to make the experiments both in the heat of 

 summer and in the cold of winter. The work was completed on Feb- 

 ruary 23, 1885. 



'" Mr. Eogers is now engaged in writing an account both of the proc- 

 esses employed in preparing and developing the dry collodion emulsion 

 plates used with the photo-heliographs in observing the transit of Venus 

 in December, 1882, and of the experiments which were executed to de- 

 termine thebest method of making pyroxyline for that purpose. When- 

 ever photographs are required which must sustain the test of accurate 

 measurement, the collodion emulsion process offers advantages so great 

 that every effort should be put forth to increase its general availability. 

 Some of our recent experiments incidentally tend in that direction, and 

 although primarily made to clear up obscure points relating to the tran- 

 sit of Venus work, it is hoped they will facilitate the application of collo- 

 dion emulsion in future operations requiring the use of photo-helio- 

 graphs. In these experiments pyroxyline has been made from flax, jute, 

 etc., as well as from cotton, which is the form of cellulose commonly 

 preferred. 



" It will be remembered that wet bromo-iodide plates, made by the 

 bath process, were used with the photo-heliographs in observing the 

 transit of Venus in December, 1874, and the question naturally arises 

 whether or not the negatives then obtained are strictly comj^arable with 

 those made u^jon dry collodion emulsion during the transit of December, 

 1882. For the definite settlement of that point recourse was had to 

 photographs of the solar spectrum. In June, 1881, a set of such photo- 

 graphs was made upon wet bromo-iodide bath plates of the kind used 

 in December, 1874, and similar sets are now being made with emulsions 

 as nearly as possible in the same condition with respect to age, etc., as 

 those actually employed in observing the transit of December, 1882. In 

 order to show clearly the progressive action of the solar rayg upon the 

 silver salts, each set begins with the shortest exposure capable of pro- 

 ducing an easily legible impression, and extends to exposures two or 

 tliree hundred times as great. The negatives have not yet been sub- 

 jected to critical examination, but the general result seems to be that, 

 while marked difierences exist in the action of the spectrum upon the 

 two classes of plates with the exposures given in the photo-heliographs, 

 the eflective rays are of substantially the same wave-length in both. 

 The transit plates of 1874 and 1882 are therefore quite comparable in 

 this respect, and there is no reason to apprehend systematic differences 

 between them depending upon atmospheric dispersion. The spectrum 

 photographs have also established the fact that the emulsion plates have 

 a degree of sensitiveness not very different from that of bath wet plates." 



The reduction of observations for time and latitude made at the vari- 

 ous United States stations in December, 1882, are now nearly completed 



