CH. /'// | FOCUS OF OBJECTIVES 189 



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 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 (# 57-58). 



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

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

 as a concave lens, or in other words like an amplifier (p. 123) , and the field is 

 smaller than when no tester is used. It is also seen that as the liquid in the 

 concavity approaches the glass in refractive index, 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 

 tbe focus. 



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

 field is larger, /. e. , the magnification less, and one would have to turn the 

 tube down instead of up to restore the focus. 



The table given below indicates the changes when using a tester prepared 

 by the Gundlach Optical Co. , and used with a 16 mm. apochromatic objective of 

 Zeiss, X4 compensation ocular, achromatic condenser, i.oo N. A. (Fig. 47): 



Tester and Liquid in the Concavity 



Size of the 

 Field 



Elevation of the Tube 



necessary to 

 Restore the Focus 



No tester used 1.825 mm.__ Standard position _. 



Whole thickness of the tester at one end, 



not over the cavity 1.85 mm. No change of focus. 



Tester with water.../. 1.075 mm.__ Tube raised 3^ mm. 



Tester with 95% alcohol 1.15 mm .... 3mm. 



Tester with kerosene 14 mm : .... 2mm. 



Tester with Gundlach Opt. Go's horn, liquid 1.825 mm. __ .... T '/ 5 mm. 



Bausch & I/omb Opt. Go's horn, liquid i 825 mm.__ .... T 2 5 (j mm. 



Leitz' horn, liquid 1.825 mm._. .... f^j mm. 



Zeiss' hom. liquid 1.825 mm. _. TSO mm - 



% 261. Equivalent Focus of Objectives and Oculars. To work out in 

 proper mathematical form or to ascertain experimentally the equivalent foci 



