136 HOW TO SEE WITH THE MICROSCOPE. 



that of Mr. Gundlach's objective, we worked it a dis- 

 tance of ten inches, and observed an amplification of 

 eighteen linear. Entering column two of the table, we 

 find at a glance eighteen to be opposite to focal length 

 of one-half inches. 



In the case of the so-called fifth, with the two micro- 

 meters at ten inches distance, we observed a magnifica- 

 tion of thirty-eight linear ; entering the second' column 

 of the table again, opposite thirty-eight, we have the 

 focal length, equal one-fourth inch. To the last com- 

 putation we worked the same case, but with the micro- 

 meters twelve and one-half inches distant ; in this case 

 observing a magnification of 48. Entering the table 

 in column third, opposite to 47.99, we have the same 

 focal length as before, viz., one-fourth inch. 



It often occurs that many first-class stands cannot be 

 worked at a distance of ten inches with objectives of 

 ordinary lengths. In this case use the twelve and one- 

 half inch distance, which can always be obtained by 

 using the draw tube. 



