MEASUREMENT OF OBJECTS. 



Since they are equally magnified, their real will be in the propor- 

 tion of their apparent magnitudes. If, therefore, they appear 

 equal, they will be equal, and if that which we desire to measure 

 appear to be twice or half the size of that whose magnitude we 

 know, its real magnitude will be twice or half that of the latter. 



Such was the micrometric method used by the earlier observers. 

 Thus Lewenhoeck procured a number of minute grains of sand, 

 sensibly equal in magnitude, and placing as many of them in a 

 line, and in contact, as extended over the length of an inch, he 

 ascertained the fraction of an inch, which expressed the diameter 

 of each. "When he desired to ascertain the actual magnitude of an 

 object seen with his microscope, he placed one of these grains 

 beside it, and estimated by comparison the magnitude of the 

 former. 



Various natural objects, whose magnitudes are known, and 

 which are subject to no perceptible variations, such as the sporules 

 of Lycoperdon bovista or puff-ball, whose diameter is the 8500th. 

 of an inch, those of the lycopodium, which measures the 940th. 

 of an inch, and others such as hair, the filaments of silk, flax, 

 and cotton, and the globules of blood, have been suggested as 

 standard measures to be similarly used. 



More modern observers, adhering to the same method, have 

 substituted artificial for natural standards. Thus extremely fine 

 wire, called micrometric wire, has been used. This wire can be 

 drawn with an astonishing degree of fineness. Dr. "Wollaston 

 invented a process by which platinum wire was produced, whose 

 thickness was only the 30000th part of an inch.* 



56. Such measurements are now more generally made by means 

 of a minute scale engraved on glass, with a diamond point. Let 

 us suppose, for example, a line, the 20th of an inch in length, 

 traced across the centre of a glass disc, set in a thin brass plate 

 of the size and form of the sliders on which objects are mounted. 

 Let this line be divided into 100 equal parts, every fifth division 

 being distinguished by a longer line, and every tenth by a still 

 longer one. Each of these divisions will be the 2000th part, 

 the intervals between the fifth divisions will be 400th, and that 

 between the tenth divisions the 200th part of an inch. This 

 microscopic scale will be seen magnified with the microscope, and 

 any microscopic object laid upon it will be seen equally magnified, 

 so that its dimensions can be ascertained by merely counting the 

 divisions of the scale included between those which mark its 

 limits when placed in different positions on the scale. 



It may perhaps be thought impracticable to make divisions so 



* Handbook of Natural Philosophy, 2d edition, Mechanics, 38. 

 E 2 61 



