122 SELECTION AND UE 



to be done. To those who are addicted to what Holmes calla 

 "fighting objectives," resolution will be the quality to be 

 desired; others will prefer penetration, flatness of field, etc. 

 In our estimation, for the purposes of ordinary scientific work, 

 we would assign to these qualities values in the following 

 order: 1. Defining power; 2. Freedom from aberration of 

 form; 3-4. Kesolution or Penetration; 5. Working distance; 

 6. Achromatism ; 7. Flatness of field. The first quality that should 

 be secured in every lens is undoubtedly defining power, and this 

 v/hether its angular aperture be high or low. Achromatism we 

 place low in the scale, because unless so marked as to injure 

 the definition, a little color does no harm. Flatness of field we 

 place last, because it will be found that perfect flatness of field 

 is very seldom combined with first rate definition. Indeed, we 

 have heard one of the moat celebrated makers of objectives 

 assert that the two qualities are to a certain extent antagonistic. 

 In giving advice in regard to the selection of an objective, 

 one of the points concerning which it is most difficult to arrive 

 at a decision, is that of angular aperture. Fortunately, however, 

 experienced microscopists may safely be left to decide this ques- 

 tion for themselves, and since those who have had no experience 

 will find it difficult to use objectives of very wide aperture, it will 

 certainly be prudent for them to choose those of moderate angle. 

 Objectives of very high angle are worthless, unless the illumina- 

 tion is well managed, and the adjustment for thickness of cover 

 properly regulated. On the other hand, a good non-adjusting 

 lens will give very fair results, with but a moderate amount of 

 skill on the part of the user. Almost all our best makers now 

 produce objectives of moderate angle, which do not adjust for 

 thickness of cover, but which have considerable resolving 

 power. We have now before us a one-fifth which costs but fif- 

 teen dollars, and which will easily resolve the Pleurosigma An- 

 gulatum by central light. An important point for consideration 

 will, of course, be, whether or not the glass is intended for 

 original work, or merely for the study or examination of well 

 known objects. The work of the physician is chiefly amongst 

 well known objects, and may be very satisfactorily accomplished 

 by means of good non-adjusting objectives, a great point in 

 favor of such glasses being that work may be done \vitU 



