OBJECTIVES OF EOUEIl BALSAM A^GLE. 145 



this is a desirable quality in a glass. It enables us to 

 search through an extemporized mount in the least pos- 

 sible time. And then, again, we are less liable to allow 

 important details of structure to escape our attention. 

 These are advantages, and must be recognized as such; 

 but, per contra, let it be remembered that in subse- 

 quent examinations it often becomes quite as important 

 that this quality of penetration should be absent. For 

 instance, when it is desired to study structures situated 

 in one plane, and one plane only, the less of penetra- 

 tion the better. I have not space to enlarge on this, 

 but let the reader not forget the fact. The glasses of 

 the lower balsam apertures are really the easiest to 

 manage, and yet are effective and adequate for a large 

 class of work. They have less power of light, and 

 hence do not "stand up" so well under high eye- 

 piecing, nor have they the same exquisite intensity of 

 definition. In a certain sense, the diminution of defini- 

 tion is very slight, indeed scarcely to be noticed (if at 

 all) when examining tolerably vigorous tests. Hence, 

 they are quite adequate for the resolution of nearly all 

 the recognized test objects when mounted dry. It is, 

 then, over exceedingly thin, faint, delicate, and trans- 

 parent tests that the higher angles assert their suprem- 

 acy. 



If the general definition as to what constitutes a 

 high-angled objective, given on a preceding page, be 

 accepted, then there remains room to discuss a class of 

 objectives of such focal lengths, nominally, as defy any 

 expert effort on the ' part of the optician to extend 



10 Microscopy. 



