72 SELECTION AND USE 



follows, omitting what printers call the " display " arrangement 

 of the words: " Microscopes constructed on scientific principles 

 magnifying 10,000 times." The microscope in question, as we 

 learned by personal examination, gives a magnifying power of 

 about one hundred diameters. Carpenter speaking upon this 

 point says: " The superficial magnifying power is of course es- 

 timated by squaring the linear ; but this mode of statement is 

 never adopted by scientific observers, although often em- 

 ployed to excite popular admiration, or attract customers, by 

 those whose interest is concerned in doing so. " We would, 

 therefore, advise our readers to look with suspicion upon any 

 concern advertising in this manner. Of course an advertisement 

 claiming a magnifying power of "10,000 areas or 100 diame- 

 ters " is unobjectionable, because both expressions are placed 

 upon an equal footing. It must also be borne in mind that 

 great though unintentional mistakes are often made by dealers 

 in stating the power of the microscopes they offer for sale. 

 Not long ago a friend told us that he had been offered a small 

 microscope having a magnifying power of 500 diameters, for 

 a moderate sum. We called to see it, taking the precaution to 

 put a micrometer and a foot rule in our pocket. By actual 

 measurement the highest magnifying power of this microscope 

 was 45 diameters ! Another instance occurs in the catalogue 

 of a well-known and honorable business house, who offer a 

 very neat and well made instrument, whose magnifying power 

 is claimed to be 350 diameters. Careful measurement of several 

 instruments, however, gave an average power of less than 200 

 diameters! Indeed it will in general be found that the magni- 

 fying power stated by dealers who do not devote their chief at- 

 tention to microscopes, is greatly over estimated. 



So much, then, being clearly understood in each case, the 

 question naturally arises, What should be the magnifying powers 

 possessed by microscopes intended for certain specified purposes? 

 That a certain magnifying power is necessary, no matter what 

 the quality of the lenses may be, is true beyond a doubt. 

 Thus, for example, suppose we wish to see the lines on the 

 Pleurosigma Angulatum, which lines are about the one fifty- 

 thousandth of an inch apart; what magnifying power woult] 

 Ibe necessary? 



