PHILOSOPHICAL SOCIETY OP WASHINGTON. 129 



Immersion objectives may be divided according to their belia- 

 vior with this apparatus into three classes: 1st. Those with 

 v/hich, since they do not have sufficient angle of aperture to ad- 

 mit the illuminating pencil, nothing can be seen, precisely as in 

 the case of dry objectives. 2d. Those v^'hich have sufficient 

 angle of aperture to admit raj^s of this obliquity, but are in- 

 capable of bringing them to an image-forming focus ; with these 

 the field appears well illuminated, but the objects are not well 

 defined. 3d. Those which not only admit rays of this obliquity, 

 but form well-defined images with them. To this class belong- 

 not merely immersion objectives with the so-called duplex fronts, 

 but others ; and I may add not merely objectives of American 

 make, but some constructed by a well-known English house. As 

 might be expected, the quality of the image formed by the direct 

 rays of the sun thrown through a pinhole at this excessive ob- 

 liquity, varies very greatly in different cases. I will state, how- 

 ever, that I have thus far found at least seven objectives, some 

 of English, others of American make, which define suflficiently 

 well under these circumstances to resolve Amphipleura pellucida 

 mounted in Canada balsam. With the objectives which per- 

 formed best, the field was of exceeding whiteness and brilliancy, 

 but by no means dazzling, the frustule undistorted and the stri® 

 clean and black on the white ground, very little color-aberration 

 being perceived. With other objectives there was more or less 

 color-aberration and distortion, both which faults were in one 

 or two cases very conspicuous, although in the part of the frustule 

 most sharply focussed upon, the strise were handsomely brought 

 out. The objectives with which I thus succeeded, ranged all the 

 way from a |th to -^V^h immersion. I will add, that the objec- 

 tives which resolved AvipMpleura pellucida under these trying 

 circumstances, when used in the ordinary way with this or other 

 test objects, displayed an exquisite perfection of definition, which 

 it would be hopeless to expect to attain with objectives of less 

 angular aperture. 



• As it is no part of my purpose in this communication to pro- 

 voke ill-tempered discussion of the merits of individual makers, 

 I will not append a list l>f the results obtained with the various 

 immersion objectives I have tried in this way. The apparatus 

 can be constructed for a few shillings, and those who take the 



