APPENDIX. 



331 



only be obtained at the expense of a great reduction of penetration or 

 focal depth, and consequently also of working distance, qualities which, 

 are essential in some of the most important kinds of Biological investiga- 



FIG. 501. 



tion; and the Author, ^nerefore, still holds to the opinion, that for 

 objectives intended to be used for such purposes, ' moderate angles ' are? 



