APPENDIX.] TESTER FOR IMMERSION LIQUID. 



213 



TESTING HOMOGENEOUS IMMERSION LIQUID. 



1 354. En order that one shall realize the fall benefit of the homogeneous im- 

 mersion principle it is necessary that the homogeneous immersion liquid should 

 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 iu which was ground accurately a small concavity and another per- 

 fectly 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 re- 

 fractive 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 iu 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 adapter 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 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 (J 46-47). 



2 355 The following table indicates the points with a tester prepared by the 

 Guudlach Optical Co., and used with a 16 mm. apochromalic objective of Zeiss, X 4 

 compensation ocular, achromatic condenser, 1.00N. A. (Fig. 41) : 



TESTER AND LIQUID IN THE 

 CONCAVITY. 



Nd tester used 



Whole thickness of the tester at 



one end. not over the cavity . . 



Tester with air in the cavity . . . 



Tester with water 



Tester with 95% alcohol .... 



kerosene .... 



GundlachOpt. Co.'s 



hoin. liquid 



Bau*ch & Lomb Opt. Co.'s horn 



liquid 



Leitz' horn, liquid 



Zeiss' hom. liquid 



SIZE OF THE 

 FIELD. 



1.825 mm. 



1.85 mm. . 

 .6 mm. . 

 1.075 tnni. 

 1. 15 mm. . 

 1.4 mm. . 



1.825 mm. 



I 825 mm. 

 1.825 mm. 

 1.825 mm. 



ELEVATION OF THE TUBE NEC- 

 ESSARY TO RESTORE THE 

 FOCUS. 



Standard position. 



No change of focus. 

 Tube raised 6 mm. 



3'A mm. 



3 mm. 



2 mm. 



TTiX mm.. 



rA mm - 

 1% mmv 



