BROAP-GUAGE OBJECTIVES, ETC. 231 



necessarily limited; we have, however, instituted a few 

 comparative tests which we now present to our readers. 



The working distance of the two-thirds worked with 

 one-fourth inch solid ocular and standard ten-inch tubes 

 is twenty-five-one-hundredths of an inch. 



The working distance of the new inch with same oc- 

 ular, worked with ten-inch tube, is thirteen-one-hun- 

 dredths of an inch ; when worked with the same eye-piece 

 and the short tube of the " Histological," the working 

 distance is increased to eighteen-one-hunclredths of an 

 inch. 



In general, the definition of the inch is superior to 

 that of the two-thirds. The very best definition of the 

 latter is obtained by the use of the full length tube and 

 one-fourth inch ocular, while the highest definition of 

 the inch is reached by the use of the short tube and 

 same ocular. 



The inch* worked over the balsamed Moller plate, gives 

 readily nice shows of the transverse striae of P. Balti- 

 cum, and with a little management both sets of lines 

 are brought out. On the G. Marina, immediately pre- 

 ceding the BaUicum, and really the more difficult shell, 

 the inch gave me a very nice stand of the transverse 

 striae from end to end of the shell, its performance over 

 this diatom being manifestly superior to that of the 

 two-thirds. 



On dry mounted shells of the P. Balticum the two 

 sets of lines are very well displayed by the inch, as are 

 also the transverse striae of dry angulatum, a favorable 

 frustule being selected. 



