20 THE AMERICAN MONTHLY [.hmuiiry, 



LKTIKHS TO THE EDinHJ. 



NoTK. ~ TAis column is of'tii to all corri-ipoiuh-nls -.vho ivrite upon the 

 topics fHMmeraled under " Problems'^ or -vfio g^i-e other in/oriiKition of 

 imtereat. The fact (hat a froblem has been ansivered once need not deter our 

 friends from makint^ additional comments. To facilitate reference, corre- 

 spondents should cite the number as well as the patrf on -vhich hiire ap- 

 peared letters and t/ueries to which reference is made. 'The editor is not 

 responsible for the ;'/V;t'.< of others published in this periodical. 



(i) Dry Objectives. In piohlcm No. i is propoundod a 

 rather hard c|ucsti(in (p. 19). In attcmptinjj^ to answer it several 

 consiilerations sugjjcst themselves. 



For the examination of diatoms, witliout attem|)tinfj to g^o into 

 their minutest studv. a fine chv 1-6 or 1-8 (sav sncli, foi" instance, 

 as the Reichert No. ja) wonlii Ik' found most «j;enerally suitable, 

 and would he more convenient tlian an immersion lens. Such a 

 lens will also do good work in tlie examination of l)acteria, 

 bacilli, etc. 



But for the most thorough study of diatoms, their finest mark- 

 ings, and the minutest bacilli, micrococci, etc., a first-class oil- 

 immersion 1-12 inch objective should be used. Such an objective 

 is undoubtedly greatly superior to a dry lens in resolving power 

 and in the clear showing of minutest structure. A dry objective 

 is more quickly and easily used, but there is nothing really diffi- 

 cult in the use of an oil-immersion lens, if properlv used. A 

 common fault is to use too much oil. A very little only is neces- 

 sary, and the use of the oil in small quantities is conducive to 

 neatness and cleanliness. Edward Pennock. 



Qi'BEN & Co., PiuLA., y(7//. /p, iHgj. 



(2) Dry Objectives. — I think all agree that the best images 

 are given bv the homogeneous objectives. They serve me the 

 best. S. 11. (jage. 



CORNEI.I. UnI\'EUSITV. 



(3) Dry Objectives. — Replying to your query, would say that 

 the definition and resolution of any dry working objective is not 

 lacking to the limit of capacity as given under N. A. for each 

 objective. A dry objective is always preferable, as it is less 

 bothersome, but its capacity (angle or aperture) is limited. It 

 very frequentlv occurs, particularly in the resolution of diflbrent 

 diatom tests, that nothing less than the best of high-power immer- 

 sion objectives must be used. Bau.sch & Lomh Opt. Co. 



Rochester, N. \.,Jan. 20^ 1^93 • 



(4) Dry Objectives. — Both classesof objectives havetheir uses. 

 For rapid work over tolerably familiar objects, a dry objective is 

 indispensable. A good 1-6 inch of 140^ will do much excel- 

 lent work. Mounts, temporary or permanent, may be quickly 



