90 SELECTION AND USE 



wide angle, viz. , that they are difficult to use by novices, does 

 not hold in the case of low powers. A good inch, of compara- 

 tively high angle, is more easily used than a poor triplet. 



A question which has considerably occupied the attention of 

 inicroscopists, is the value of objectives of high power, and their 

 efficiency as compared with those of lower denominations. 

 That in many cases considerable amplification or magnifying 

 power is absolutely necessary, admits of no doubt; but the ques- 

 tion to be settled is: suppose that we wish a power of 2,000 diame- 

 ters, would it be better to get this by means of a tenth of an inch 

 objective, magnifying 100 times, and a half inch eye-piece mag- 

 fying 20 times, or by a twentieth of an inch objective magnify- 

 ing 200 times, and an inch eye-piece magnifying ten times ? 



It is not very many years ago since one of our ablest Ameri- 

 can objective makers held that a lens of a quarter of an inch 

 focus might be made to do anything that a lens of any power 

 could be made to do, and the ground of this opinion was that 

 the individual lenses of objectives as low as a fourth, could be 

 made so much more perfect than the smaller lenses of higher 

 powers, that this perfection more than counterbalanced the 

 greater magnifying power of the objective of shorter focus. 

 The reasoning here seems sound and obvious, but it has been 

 found in practice that for everything except resolution, the limit 

 to which the power of objectives may be carried, is far beyond 

 a fourth. For resolution it has, we believe, been found that a 

 well made tenth is capable of doing anything that any lens can 

 do; for other kinds of work sixteenths and twenty-fifths, and 

 even fiftieths and eightieths have been declared to possess 

 advantages that are obvious. This, however, is one of those 

 points upon which authorities differ; Beale, for example, favors 

 high powers; Carpenter and Frey seem inclined to think that 

 very high powers show nothing that cannot be seen by means 

 of objectives of gleater focal length. 



French objectives of the numbers 1, 2, 3 and 4, if carefully 

 selected, are capable of doing really serviceable work. A few 

 years ago, some of the best known makers of American micro- 

 scopes used nothing else, even in microscopes costing $150, but 

 this course we can scarcely regard as judicious, for whenever 

 the microscopist is prepared to expend $75 or more for a micro- 



