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 

 istwenty-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-hundredths 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 balsam ed Moller plate, gives 

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

 cum, and with a little management both ssts of lines 

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

 ceding the Balticum, 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 beiii selected. 



