140 PROCEEDINGS OF THE AMERICAN ACADEMY 



" But this is not the first example which has occurred to me. As 

 far back as a year ago last summer I visited Mr. Spencer, and spent 

 several days with him in testing his objectives with the tissues of every 

 creature which we could find. I shall never forget the astonishment 

 and delight with which I occupied day after day, plunged into the hith- 

 erto unknown depths of organic life. I say this after having tested 

 from time to time some of the best English microscopes which have 

 been made since the ' Great Exhibition,' and therefore am not to be 

 supposed to have made so great a leap as if from an Oberhaeuser to a 

 Spencer. Since that visit, and another one also, made last summer, 

 when I obtained one of Mr. Spencer's quarter-inch objectives, with an 

 angular aperture of one hundred and forty-five degrees, I have from 

 time to tune made particular efforts to test the value of the flat field 

 and wide angle in the study of organized bodies. The results of my 

 investigations at Canastota, and also since my return, I have embodied 

 in this paper. 



"One of the most valuable properties of the flat field is, that it 

 enables one to study an isolated cell, in a manner totally unexpected 

 to me, making it possible to obtain a section of such a body at any 

 horizon, as if it were actually cut across. As I have said before, the 

 flat field ignores everything above and below its horizon, and there- 

 fore, if it is brought on a level with the equator of a spherical cell, the 

 largest possible circle is obtained, and the actual thickness of the wall 

 becomes apparent ; and if it is raised or lowered, the circle grows 

 smaller and the wall seems thicker, because of the obliquity of the 

 section, and yet appears as distinct as the one at the equator. This 

 may go on until the field approaches very closely to the upper or lower 

 side, and then the inner surface of the cell appears. In an ordinary 

 microscope, the far-reaching power of the objective utterly precludes 

 the possibility of such a process of investigation. 



" The relations of the Purkinjean vesicle to the yolk, and the num- 

 ber and position of the Wagnerian vesicles, have always been difficult 

 subjects to work out Avith the ordinary microscope. If the Wagnerian 

 vesicle was situated at the upper or lower side of the Purkinjean vesi- 

 cle, it has often been next to impossible to tell whether it might be 

 really within the latter, or was one of the very similar yolk-cells out- 

 side. There are many other instances of the like kind too numerous 

 to mention. All this difficulty I have seen obviated by the decided, 



