THE WOODWARD ILLUMINATOR. 16 9 



nothing can be seen, precisely as in the case of dry ob- 

 jectives. 2d. Those which have sufficient angle of aper- 

 ture to admit rays of this obliquity, but are incapable 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 ob- 

 jectives 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 pin hole 

 at this excessive obliquity varies very greatly in differ- 

 ent cases. I will state, however, that I have thus far 

 found at least seven objectives, some of English, others 

 of American make, which define sufficiently well under 

 these circumstances to resolve Amphipleura pellucida 

 mounted in Canada balsam. With the objectives which 

 performed best, the field was of exceeding whiteness 

 and brilliancy, but by no means dazzling, the frustule 

 undistorted, and the striae clean and black on the white 

 ground, very little color aberration being perceived. 

 With other objectives there was more ot less color aber- 

 ration 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 striae svere 

 handsomely brought out. The objectives with which I 

 thus succeeded ranged all the way from one-fourth to 

 one-sixteenth immersion. I will add that the objectives 



