168 HOW TO SEE WITH THE MICROSCOPE. 



a beam of parallel solar rays (white sunlight), reflected 

 from a plane mirror, be thrown through the two aper- 

 tures upon the face of the prism, being perpendicular 

 to that face, it will enter and pass through without 

 refraction until it reaches the upper surface of the thin 

 glass cover Gr. The parallel rays impinge upon this 

 surface, as is evident from the construction, at an angle 

 of 45 with the optical axis O O. If, now, the medium 

 next above the thin cover, Gr, be air, this obliquity will 

 be greater than the critical angle, and total reflection 

 of the rays will take place. If, however, the medium 

 next above the thin cover be water, the obliquity will 

 not be greater than the critical angle. Refraction hav- 

 ing taken place, the rays will enter the water, H ; and 

 if an immersion lens of sufficient angle of aperture be 

 focussed upon the objects mounted beneath the cover Gr, 

 these rays not merely enter the front of the objective, but 

 will form a well-defined image of the object on a brightly 

 illuminated field, which will be visible through the eye- 

 piece of the instrument in the usual way. Of course it 

 is evident from the diagram that with no dry objective, 

 or any immersion objective of less than 90 balsam angle, 

 can anything whatever of balsam-mounted objects* thus 

 be seen. 



" Immersion objectives may be divided according to 

 their behavior, with this apparatus, into three classes: 

 1st. Those with which, since they do not have sufficient 

 angle of aperture to admit the illuminating pencil, 



* The apparatus can be used, of course, to secure black-ground illumina- 

 tion of suitable dry objects if they are mounted on the slide instead of the 

 cover, as is usual. 



