50 



THE AMERICAIS MONTHLY 



[March, 



An Ideal Series of Objectives for 

 Microscopical Work.* 



[Governor J. D. Cox, of this city, 

 recently wrote a leading microscopist 

 on the subject of an ideal series of 

 objectives for microscopical work. 

 His correspondent, regarding the 

 views of value, submitted them, we 

 understand, to Prof. Abbe, of Jena, 

 hoping he might make formulae for 

 such a series, to be worked out under 

 his supervision by Zeiss. At our re- 

 quest, the Governor has given us that 

 portion of the letter in which he dis- 

 cusses the subject for publication, 

 which we here present to our readers. 

 — Ed. Med. News?^ 



" Prof. Abbe's enunciation, in a re- 

 cent article, of the proposition, in 

 substance, that one great objective 

 of a given angle of aperature should, 

 if properly constructed, do all that 

 any glass of that aperture will do, 

 strikes me with peculiar force, be- 

 cause I have been leaning to that view 

 myself. It amounts to this, viz. : An- 

 gle of aperture determines the power 

 of discriminating minute variations of 

 structure, or of surface in the plane 

 which is in focus ; ergo, if the object- 

 ive is thoroughly well corrected, eye- 

 piecing will do nearly all that increase 

 of power in the objective would do, 

 without increasing or widening the 

 angle. 



"A practical suggestion grows out 

 of this, which, if it could be carried 

 out by Zeiss under Prof. Abbe's direc- 

 tions and with his formulae, would be 

 of interest to the microscopical world. 

 It is, that the true model for a series 

 of objectives, would be a list contain- 

 ing the lowest powers that can be 

 thoroughly well made of each of the 

 desirable angles which will combine 

 maximum aperture with low magnifi- 

 cation. The series would thus be sci- 

 entifically progressive. AVe should 

 then only need to select those which 

 would combine the most desirable 

 working -distance with other qualities, 



*From Cincinnati Medical News, January 

 1881. 



and the model series would be com- 

 plete. 



"Besides the lowest powers, we 

 should want one objective of 40° an- 

 gle for use with the binocular upon 

 opaque objects. We should want a 

 glass, v/ith nearly half an inch actual 

 working-distance, for use with dissect- 

 ing instruments or with the mechanic- 

 al finger. Another, with an eighth or 

 a tenth of an inch clear working-dis- 

 tance, would be needed for rough ex- 

 aminations of algae, etc., in the com- 

 mon animalcule cage or compressor, 

 with pretty thick cover glasses. The 

 problem is. What is the highest angle 

 consistent with these conditions ? 

 Indeed, I do not see why we should 

 not rate our glasses by the angle of 

 aperture rather than by the so-called 

 focal distance, for we should always 

 know what a thoroughly corrected 

 glass of a given angle ought to do ; 

 whereas, nobody knows what a glass' 

 performance will be, because it is 

 called a "quarter" or an "eighth," 

 under the present nomenclature. 



"In further elucidation of the mat- 

 ter, it may be well to refer to the 

 tables which the Royal Microscopical 

 Society have publised and kept stand- 

 ing in their journal. These tables 

 are based upon Prof. Abbe's notation, 

 and show the resolving and defining 

 power of objectives of various angles, 

 theoretically calculated from Abbe's 

 formulae. They give, of course, the 

 possible performance of glasses, to 

 which objectives will approximate ac- 

 cording to the perfection of their cor- 

 rections and finish. It is easy to see 

 how a series of glasses, constructed 

 upon the conditions which have been 

 stated above, may combine maximum 

 performance in each department of 

 work, with the minimum number of 

 objectives. Thus, the scientific out- 

 fit of the microscopist would be made 

 at once, least cumbersome in quantity 

 and most efficient in quality. In this 

 way, we should have what might fair- 

 ly be called an ideal series of lenses. 



In practice, the result would be 

 somewhat as follows, viz. : i. An ob- 



