NATURAL SCIENCES OF PHILADELPHIA. 129 



connection with the reproduction of the structure of the trap 

 rocks as exhibited in thin slices. Among the causes which had 

 hindered the application of micro-photography to this purpose was 

 the fact that the constituent minerals which were colored yellow 

 or red became black in the photograph. The experiment of pho- 

 tographing the slides in polarized light and in different positions 

 of the analyzer has met with very fair success. 



The photographs of the enlargements of 34 and 13G diameters, 

 respectively, had been exhibited in connection with a paper 

 read before the last meeting of the American Philosophical 

 Society, but at that time the expedient of overcoming the anacti- 

 nism of parts of the image was merely suggested. Since then 

 six photographs of the same part of the same thin section of the 

 dolerite, best known, have been made. In five of these photographs 

 all the conditions remained the same except the position of the 

 analyzei", which was rotated to five points corresponding to max- 

 ima and minima of some of the more prominent objects in the 

 field. A particular crystal of pyroxene being selected, it was 

 pointed out that in three of the photographs this was light colored 

 with a dark axis, and in two dark with a light axis. Thus in a 

 series of photographic prints made in this wa} r the minerals which 

 were dark from the anactinism of their transmitted light were 

 easily distinguished from those that were dark from opacity, as 

 for example magnetite. 



As a means of still further modifying the conditions, No. 6 was 

 photographed just like No. 4, except that a selenite plate was in- 

 troduced over the slide and between the prisms. Prof. Frazer 

 concluded b}' remarking that he had delayed returning the thin 

 slides of Connecticut traps prepared and kindly loaned to him by 

 Mr. E. S. Dana, of New Haven, for exhibition before the Philo- 

 sophical Society, until he had repeated the experiments before the 

 Academy. He informally invited those members to remain after 

 the present meeting who desired to see the slides in polarized 

 light. The Connecticut and Pennsylvania traps were then pro- 

 jected on the screen and their effect on polarized light illustrated. 



Ellicott Fisher and J. S. Alexander were elected members, and 

 Col. T. M. Bryan, of Vincenttown, N. J., was elected a correspond- 

 ent. 



The committees to which they had been referred recommended 

 the following papers to be published : 



