188 TEST PLATE AND APERTOMETER [CH. VII 



the center of the circle. To use the apertometer the microscope is placed in 

 a vertical position, and the perforated circle is put under the microscope and 

 accurately focused. The circular edge of the apertometer is turned toward a 

 window or plenty of artificial light so that the whole edge is lighted. When 

 the objective is carefully focused on the perforated circle the draw-tube is 

 removed and in its lower end is inserted the special objective which accom- 

 panies the apertometer. This objective and the ocular form a low power com- 

 pound microscope, and with it the back lens of the objective, whose aperture 

 is to be measured, is observed. The draw-tube is inserted and lowered until 

 the back lens of the objective is in focus. "In the image of the back lens 

 will be seen stretched across, as it were, the image of the circular part of the 

 apertometer. It will appear as a bright band, because the light which' enters 

 normally at the surface is reflected by the bevel part of the chord in a vertical 

 direction so that in reality a fan of 180 in air is formed. There are two sliding 

 screens seen on either side of the apertometer; they slide on the vertical circu- 

 lar portion of the instrument. The images of these screens can be seen in the 

 image of the bright band. These screens should noiv be moved so that their 

 edges j list totich the periphery of the back lens. They act, as it were, as a 

 diaphragm to cut the fan and reduce it, so that its angle just equals the aperture 

 of the objective and no more." " 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 determine 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 objec- 

 tive 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. 



\ 260. Testing Homogeneous Immersion Liquid. In order that one 

 may realize the full benefit of the homogeneous immersion principle 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. Ainer. 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 opti- 

 cally 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 



