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niMcromctr^.* 



By E. G. Love. Plate XV. 



WHOEVER undertakes any serious work with the micro- 

 scope is confronted, sooner or later, with the necessity 

 for some method of determining the actual size of 

 microscopic objects; and it may not be without profit if we briefly 

 review the different methods for ascertaining this. 



As we might naturally suppose, the earliest efforts in micro- 

 metry consisted in comparing the microscopic object with other 

 objects whose approximate size was known or could easily be 

 determined. Thus Leeuwenhoek employed grains of sea-sand 

 of such size that one hundred of them, placed side by side, 

 extended one inch. One of the sand grains was so placed that it 

 could be directly compared with the object under the microscope. 

 The same idea was adopted by later microscopists who employed 

 the very small spores of certain plants. 



Hooke, as early as 1675, described a method for determining 

 the magnifying power of a microscope. To use his own words : 

 " Having rectified the Microscope, to see the desired Object 

 through it very distinctly ; at the same time that I look upon the 

 Object through the Glass with one Eye, I look upon other Objects 

 at the same Distance with my other bare Eye : by which means I 

 am able, by the Help of a Kuler divided into Inches and small 

 Parts, and laid on the Pedestal of the Microscope, to cast as it 

 were the magnified Appearance of the Object upon the Ruler, and 

 thereby exactly to measure the Diameter it appears of through 

 the Glass ; which, being compared with the Diameter it appears 

 of to the naked Eye, will easily afford the Quantity of its being 

 magnified." While Hooke referred to the determination of the 

 magnifying power, and that with objects capable of being measured 

 without the aid of the microscope, the same method is applicable 

 to ordinary micrometer work. 



Dr. James Jurin wound a piece of very fine silver wire about 

 a pin, leaving no space between the threads, measured any length 



* Substance of some remarks made before the New York Microscopical 

 Society. From the Journal of the New York Microscopical Society. 



