30 The Solar Eclipse of August last. [Jan., 



E. L. Wilson, H. 0. Phillips, E. Moelling, J. C. Browne, W. J. 

 Baker, James Cremer, H. W. Clifford, 0. H. Kendall, J. Mahoney, 

 and W. Y. Eanger. 



It was a question of some moment to decide whether, for 

 obtaining the photographic records, they should follow the plan 

 adopted by the French and German expeditions of last year, and 

 take the photograph in the principal focus of the object-glass, thus 

 securing great intensity of light in a small image, or follow the 

 method employed by Dr. De la Kue in 1860, when he used an 

 ordinary Huygenian eye-piece so placed as to produce an enlarged 

 image of the first image from the object-glass. It was found by 

 experiment that with a clear sun it was necessary to reduce the 

 aperture of the telescope (which was 4 inches, and 50 inches focus) 

 to lj- inch, and to use a diaphragm slide of ^ftth inch aperture, 

 in order to get a proper exposure when the solar image was 

 enlarged from • 6 inch (its diameter at the principal focus of the 

 objective) to 2J inches on the ground glass. The same size of 

 aperture was adopted for the larger instruments during the partial 

 phases, the entire aperture, in all cases, of course being used 

 during totality. 



The work of designing and constructing these lenses, and also 

 the different attachments to the cameras for securing exposures of 

 various degrees of rapidity, from a very small fraction of a second 

 to any desired length, was placed in the hands of Mr. Joseph 

 Zentmayer, whose extended scientific attainments, combined with 

 unrivalled skill in the construction of optical instruments, peculiarly 

 fitted him for such a task. 



As the operation of the eye-piece, when employed to produce an 

 image on the screen or ground glass of a camera, is essentially 

 different from that which it performs in its usual office, it was 

 judged best by Mr. Zentmayer to make some alterations in its 

 form. Thus, in the first place, since in the present case the " eye- 

 lens " of the eye-piece undoubtedly makes a secondary image of the 

 primary image formed within the eye-piece by the combined action 

 of the objective and the field-lens of the eye-piece, it is clearly 

 desirable to make this lens of a longer focus than usual, so that its 

 errors may be of less account. It was also essential to give the 

 new eye-piece a wide angle, so as to secure a sufficient field not only 

 for the solar disc, but also for the corona. 



While therefore the ratio of focal lengths in the two lenses of 

 the ordinary eye-piece is usually 1 : 3, it was in this case as 1:2. 

 While the distance between the lenses is usually the sum of their 

 focal lengths divided by 2, it was here made equal to the sum of 

 the focal lengths divided by 2, plus • 24 inch. This was to give 

 space for the introduction of the reticule of spider lines, which 

 would otherwise have been brought too near the field-lens, and also 



