108 HOW TO SEE WITH THE MICROSCOPE. 



angle, but is also a severe test as to the general quali- 

 ties of an objective?" 



In response to this interrogatory, I reply, that it's 

 high time things were " screwed down." As to the 

 latter portion of the remark of my friend, it may be 

 observed that the directions given indicate a part, and 

 part only, of the course to be pursued in testing the 

 performance of a really first-class American objective 

 of wide aperture. 



The plan proposed is liable to another, and, in the 

 minds of some, a most serious objection; says one, 

 " Don't you see that you have advanced no positive 

 guage? Your idea simply is to compare one glass w r ith 

 another in short, it means 'fighting objectives" 

 Selah! 



SOMETHING FURTHER ABOUT OBJECTIVES. 



There is another matter of common acceptance which 

 has in the past made some mischief; I refer to the fact 

 that the focal or working distance of an objective has 

 been and is considered the index of its capacity for cer- 

 tain classes of work. Thus, the incii has been set apart 

 for the study of such objects as required examination, 

 with powers from 50 to 150 diameters, and where its 

 comparatively long working distance was desirable, 

 while the one-fiftieth, whose lowest power of 2,500 

 diameters and its exceedingly short working distance, 

 could not perform the work of the inch the one- 

 fiftieth being reserved for the investigation of the most 

 minute organisms, and under the highest amplifications. 



