20 THE AMERICAN MONTHLY [January, 



LETTERS TO THE EDITOR. 



Note. — This coluiiifi is open to all correspondents who tvrite upon the 

 topics enumerated itnder ^^ Problems,^'' or -who give other ififormation of 

 interest. The fact that a problem has been a?isivered once ?ieed not deter our 

 frie?ids fron making additional comments. To facilitate refereiice^ corre- 

 spondoits should cite the ntimber as -well as the page on ivhich have ap- 

 peared letters and queries to which reference is made. The editor is not 

 responsible for the views of others published in this periodical. 



(r) Dry Objectives. - In problem No. i is propounded a 

 rather hard question (p. 19). In attempting to answer it several 

 considerations suggest themselves. 



For the examination of diatoms, without attempting to go into 

 their minutest study, a fine dry 1-6 or 1-8 (say such, for instance, 

 as the Reichert No. 7a) would be found most generally suitable, 

 and would be more convenient than an immersion lens. Such a 

 lens will also do good work in the examination of bacteria, 

 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 I- 1 2 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 properly 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. 



Queen & Co., PHiLA.,ya;;. /p, iSgj. 



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



are given by the homogeneous objectives. They serve me the 

 best. S. H. Gage. 



Cornell University. 



(3) Dry Objectives, — Replyingtoyour 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 frequently occurs, particularly in the resolution of diflerent 

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

 sion objectives must be used. Bausch & Lomb Opt. Co. 



Rochester, N. \.^Jan. 20^ iSgj. 



(4) Dry Objectives. — Both classesof objectives have their 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 



