CH.X~\ TEST PLATE AND APERTOMETER 271 



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 an ob- 

 jective 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. 



\ 431. Testing Homogeneous Immersion Liquid. — In order that one shall 

 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. Amer. Micr. Soc. ; 1885, p. 83). It will be readily seen that 

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

 will act as a concave or dispersing lens. If filled with a liquid of greater refractive 

 index than glass, the concavity would act 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 would be 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 ob- 

 scured. An adapter with society screw is put on the microscope and the objective 

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

 and depth to receive the tester which is just above the objective. As object it 

 is well to employ a stage micrometer and to measure carefully the diameter of the 

 field without the tester, then with the tester far enough inserted to permit of the 

 passage of rays through the glass but not through the concavity, and finally the 

 concavity is brought directly over the back lens of the objective. This can be 

 easily determined by removing the ocular and looking down the tube. 



Following Professor Smith's directions it is a good plan to mark in some way 

 the exact position of the tube of the microscope when the micrometer is in focus 

 without the tester, then with the tester pushed fc in just far enough to allow the light 

 to pass through the plane glass and finally when the light traverses the concavity. 

 The size of the field should be noted also in the three conditions ( \ 50-52. ) 



It will be seen by glancing at the following table that whenever the liquid in the 

 tester is of lower index than glass, that the concavity with the liquid acts as a 

 concave lens, or in other words like an amplifier (p. 109), and the field is smaller 

 than when no tester is used. It will also be seen that as the liquid in the concav- 

 ity approaches the glass in refractive index that the field approaches the size 

 when no tester is present. It is also plainly shown by the table that the greater 

 the difference in refractive index of the substance in the concavity and the glass, 

 the more must the tube of the microscope be raised to restore the focus. 



If a substance of greater refraction than glass is used in the tester the field 

 would be larger, i. e., the magnification less, and one would have to turn the tube 

 down instead of up to restore the focus. 



