324 The Microscope. 



objective of wide aperture, is, on this account, especially supe- 

 rior, wlien the illumination is a narrow beam of axial light? 

 When the object is too small or too slender to be seen by a 

 narrower angle, no doubt can exist even in this case of the es- 

 sential advantage of the greater aperture, but unless one wishes 

 to see the flagellum of a Bacillus, or the minute structure of a 

 Diatom valve, his laboratory work may, perhaps, be just as suc- 

 cessful with first-class objectives, of less than the widest angle 

 procurable. Should these of moderate angle possess better defi- 

 nition (not resolution), then for their proper work they are 

 better lenses. My later purchases for students' ordinary use, 

 have been of 110° air angle for a Hh inch, with the expectation 

 that anything up to the widest numerical aperture ma}'^ some- 

 times be accessible. For the closest possible studies upon the 

 exact size (measurement) and shape of small stained bacteria, 

 a Tolles' Ath inch homogeneous immersion of 123^ balsam angle 

 is the best 1 have used, though others at hand have considerably 

 wider aperture. 



GET THE BEST. 



Having decided what is most suitable for the work proposed, 

 the very best should be selected for students' use, as well as for 

 special investigators. It may be said that the expense would often 

 be too great, and that cheap instruments or none constitute the 

 alternative. Often, however, this is the mere outgrowth of too 

 cheap ideas, either on the part of the instructors or boards of 

 trustees. If the real needs are fairl}' appreciated, in this as in 

 any other case, they can usually be met in some waj'. Otherwise 

 how are microscopes obtained at all? At any rate, instructors 

 should inform themselves with the utmost care, and then equip 

 their pupils in the best possible manner, with this, the most 

 delicate of all tools. No questions of home or foreign manufac- 

 ture, of accidents of popular approval or of hereditary service, 

 should be allowed weight in the selection of a microscope ob- 

 jective. Neither should the cost price be taken as an index of 

 quality. No one can be blamed for buying what he finds to be 

 the best goods, for the least money. 



Governed b}' tliese principles, I have ceased ordering from 

 abroad for students' use. Without naming other makers, I choose 

 the objectives of the Bauseh & Lomb Optical Co., in preference 

 to those of Leitz. I have in daily use some first class wide 



