272 TEST PLATE AND APERTOMETER CH. A'] 



The table given below indicates the points with a tester prepared by the Gund- 

 lach Optical Co., and used with a 16 mm. apochromatic objective of Zeiss, ,/\ 

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



Size of the I Elevation of the Tube 

 Tester and Liquid in the Concavity ' ... necessary to 



Field Restore the Focus 



No tester used 1.825 mm. _. Standard position. 



Whole thickness of the tester at one end, 



not over the cavity 1.S5 mm. ___ No change of focus. 



Tester with water 1.075 "»"• --.Tube raised 3J2 mm. 



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



Tester with kerosene 1.4 mm. .... 2 mm. 



Tester with Gundlach Opt. Co's'hom. liquid t.825 mm. _. .... ,'„"„ mm. 



Bausch & Lomb Opt. Co.'s hum. liquid .1.825 mm. __ . • ■ ■ $5 mm - 



Leitz' hom. liquid [.825 mm. _. .... ,-„"„ mm. 



Zeiss' hom. liquid 1.825 mm. __ .... ,-„"„ mm. 



\ 4^2. Equivalent Focus of Objectives and Oculars. — To work out in proper 

 mathematical form or to ascertain experimentally the equivalent foci of these 

 complex parts with real accuracy would require an amount of knowledge and of 

 apparatus possessed only by an optician or a physicist. The work may be done, 

 however, with sufficient accuracy to supply most of the needs of the working 

 microscopist. The optical law on which the following is based is : — "The size of 

 object and image varus directly as their distance from the center oj the lens." 



By referring to Figs. 14, 16, 21, it will be seen that this law holds good. 

 When one considers compound lens-systems the problem becomes involved, as the 

 centre of the lens systems is not easily ascertainable hence it is not attempted, 

 and only an approximately accurate result is sought. 



\ 433. Determination of Equivalent Focus of Objectives. — Look into the 

 upper end of the objective and locate the position of the back lens. Indicate the 

 level in some way outside of the objective. This is not the center of the object- 

 ive but serves as an arbitrary approximation. Screw the objective into the tube 

 of the microscope. If a Huygenian ocular is used with the ocular micrometer, 

 screw off the field lens and use the eye-lens only. If a positive ocular is used 

 no change need be made. Pull out the draw-tube until the distance between the 

 ocular micrometer and the back lens is 250 millimeters. Use a stage micrometer as 

 object and focus carefullv. Make the lines of the two micrometers parallel 

 (Fig. 10S). Note the number of spaces on the ocular micrometer required to 

 measure one or more spaces on the stage micrometer. Suppose the two microm- 

 eters are ruled in /,„ mm. and that it required 10 spaces on the ocular micrometer 

 to enclose 2 spaces on the stage micrometer, evidently then 5 spaces would cover 

 one. The image, A'B' Fig 21 in this case is five times as long as the object, A,B. — 

 Now if the size of object and image are directly as their distance from the lens it 

 follows that as the size of object is known ( ,-„ mm. ), that of the image directly 

 measured ( j ]J mm.), the distance from the lens to the image also determined in 

 the beginning, there remains to be found the distance between the objective and 

 the object, which will represent approximately the equivalent focus. The general 

 formula is, Object, O: Image, I : : equivalent focus, F 1250. Supplying the known 



