128 BULLETIN OF THE 



blackened brass screen held in position by two brass arms, one 

 of which is shown in the figure. This screen is parallel to the 

 adjacent face of the glass prism, and has in it a small circular 

 aperture, E, about the size of a large pinhole. The side of the 

 glass prism next the screen is covered with black paper in which 

 is a corresponding pinhole. The two pinholes are so placed 

 that a beam of parallel white sunlight (r) passing through both 

 will be perpendicular to the side of the glass prism on which it 

 impinges. 



To use this apparatus, it is adjusted in the sub-stage of the 

 microscope, a drop of oil of cloves is placed on the upper face 

 of the prism, the glass slide F F, on which the object is mounted 

 in Canada balsam under the usual thin cover, G, is placed on the 

 stage, and the sub-stage is racked up until the drop of oil of 

 cloves is spread out into a thin layer, /, 



The object being thus arranged, it is evident that if a beam 

 of parallel solar rays (white sunlight), reflected from a plane mir- 

 ror, be thrown through the two apertures upon the face of the 

 prism, 'being perpendicular to that face, it will en*,er and pass 

 through without refraction until it reaches the upper surface of 

 the thin glass cover, G. The parallel rays impinge upon this 

 surface, as is evident from the construction at an angle of 45° 

 with the optical axis, 0. If, now, the medium next above the 

 thin cover, G, be air, this obliquity will be greater than the criti- 

 cal 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 

 having 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, G, 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 

 anythisg whatever of balsam-mounted objects* thus illuminated 

 be seen. 



* The apparatus can be used, of course, to secure black-grouurl illumi- 

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

 of the cover, as is usual. 



