SOMETHING FURTHER ABOUT OBJECTIVES. 10 ( J 



Now, in the instance cited the popular idea is correct. 

 The inch cannot do the work of the fiftieth, nor can the 

 fiftieth do the work of the inch, and each, as to the 

 other, are to be appropriately brought into use. 



But, as is well-known, between and intermediate to 

 the scope of the two glasses named, there are several 

 objectives having not only intermediate but variable 

 focal lengths. Among these latter, too, are to be 

 found the objectives known as " medium powers," and 

 it is with reference to these that it may be affirmed that 

 the broad rule governing the inch and the fiftieth does 

 not hold good. 



Some four or five years since, the author, in writing 

 to a brother microscopist, hazarded the statement that 

 the time would surely come when the optician would 

 furnish one-sixths, capable of performing all the work 

 then done with the one-fiftieth. The principal reason 

 advanced at that date, in support of his opinion, was,. 



First, assuming the case of a perfect objective with a 

 perfect eye-piece, he claimed that it made no difference 

 to which end of the tube the power should be applied. 



Second, the nearer perfection arrived at in the con- 

 struction of the objective and eye-piece, the higher may 

 be the power of the latter ; and 



Third, as we have no right to expect absolute per- 

 fection in the construction of objectives, it nevertheless 

 seemed reasonable to infer that the optician could bet- 

 ter handle and adjust a lens of sensible dimensions, 

 such as are used in the manufacture of the medium 



