668 Transactions of the Society. 



other preparations, when the illumination is effected by means of a 

 central pencil of light from a coal-oil lamp, is much improved if an 

 immersion condenser of apei-ture equal to that of the objective 

 be substituted for the ordinary dry achromatic condenser. 



But notwithstanding these notable and practically important 

 facts, it is nevertheless equally true that without these valuable 

 accessories the new immersion objectives greatly excel any dry 

 objectives in defining power, both by oblique and central light, 

 provided only that the objects examined are mounted in Canada 

 balsam, or if dry, are adherent to the under surface of the glass 

 cover. Under the same circumstances also, immersion objectives 

 whose balsam angle exceeds 100^, excel in definition immersion 

 objectives of smaller angle. Thus, several of the immersion objec- 

 tives in the Museum collection which have an aperture greater than 

 100° balsam, will resolve the 19th band of the Nobert's plate by 

 lampHght thrown obliquely upon the under surface of the slide by 

 an ordinary small bull's-eye condenser, a feat which no immersion 

 objective of smaller angle, and no dry objective, can be made to 

 perform ; and the superiority of the definition of these wide-angled 

 objectives, when used to examine histological preparations, bacteria, 

 and the like, illuminated with central light, by the ordinary dry 

 achromatic condenser, is readily recognizable by the practised eye. 



These easily verified observations are quite in accordance with 

 the accepted theory of microscopical vision. It has long been prac- 

 tically known, in the case of dry objectives, that those of great 

 aperture exhibit to a marked degree the superior defining power 

 thence resulting, even when the object is illuminated with perfectly 

 central light ; and the well-known theoretical explanation of this 

 phenomenon is equally applicable to the case of immersion objec- 

 tives so long as no film of air intervenes between the object and the 

 front of the objective. 



But the superior defining power of wide-angled immersion 

 objectives, when illuminated by central or moderately oblique light, 

 is still more manifest when monochromatic sunhght is employed ; 

 and this fact is strikingly illustrated by the photographs which 

 accompany the present paper. A converging pencil of monochro- 

 matic sunlight, obtained by a 3-inch objective of about 12° aperture, 

 with its optical axis inchned at an angle of but 45" to the optical 

 axis of the Microscope, will be found quite oblique enough to give 

 magnificent resolution of Amiohifleura pellucida mounted in Canada 

 balsam, without the use of any immersion illuminator, although in 

 this case it is evident that in consequence of refraction at the under 

 surface of the glass slide, the axis of the illuminating pencil 

 actually impinges upon the frustule at an angle of little more than 

 27° to the optical axis of the Microscope. If, now, with the 

 illuminating pencil rigidly limited to tliis angle, a selected frustule 



