Ch. IX] TESTING HOMOGENEOUS LIQUID 299 



"This angle is now determined by the arc of glass between the screens; 

 thus we get an angle in glass the exact equivalent of the aperture of 

 the objective. As the numerical apertures of these arcs are engraved 

 on the apertometer they can be read off by inspection. Nevertheless 

 a difficulty is experienced, from the fact that it is not easy to deter- 

 mine the exact point at which the edge of the screen touches the 

 periphery of the back lens, or as we prefer to designate it, the limit of 

 aperture, for curious as the expression may appear we have found at 

 times that the back lens of the objective is larger than the aperture 

 of the objective requires. In that case the edges of the screen refuse 

 to touch the periphery." 



In determining the aperture of homogeneous immersion objectives 

 the proper immersion fluid should be used as in ordinary observation. 

 So, also, with glycerin or water immersion objectives. 



§ 473. Testing Homogeneous Immersion Liquid. — In order that 

 one may realize the full benefit of the homogeneous immersion prin- 

 ciple it is necessary that the homogeneous immersion liquid shall be 

 truly homogeneous. In order that the ordinary worker may be able 

 to test the liquid used by him, Professor Hamilton L. Smith devised 

 a tester composed of a slip of glass in which was ground accurately a 

 small concavity and another perfectly plain slip to act as cover. 

 (See Proc. Amer. Micr. Soc, 1885, p. 83.) It is readily seen that this 

 concavity, if filled with air or any liquid of less refractive index than 

 glass, acts as a concave or dispersing lens. If filled with a liquid of 

 greater refractive index than glass, the concavity acts like a convex 

 lens, but if filled with a liquid of the same refractive index as glass, 

 that is, liquid optically homogeneous with glass, then there is no effect 

 whatever. 



In using this tester the liquid is placed in the concavity and the 

 cover put on. This is best applied by sliding it over the glass with the 

 concavity. A small amount of the liquid will run between the two 

 slips, making optical contact on both surfaces. One should be careful 

 not to include air bubbles in the concavity. The surfaces of the glass 

 are carefully wiped so that the image will not be obscured. An adapter 

 with society screw is put on the microscope and the objective is attached 

 to its lower end. In this adapter a slot is cut out of the right width 



