46 GENERAL HISTORY OF 



SECTION XVII. On Micrometers, and the Method of 

 Measuring Infusoria. 



The late Dr. Goring, in the Micrographia, has de- 

 scribed the method by which, in various ways, a correct 

 admeasurement may be taken of these minute creatures, 

 as also Mr. Bauer, in a paper in the same publication. 

 I cannot do better than refer the reader to these autho- 

 rities, for the fullest information attainable on this subject. 

 A few words, however, may be said on the mode of pro- 

 ceeding which I have myself adopted, and which, after 

 much practice, has been productive of very accurate 

 results. It is as follows: Having set up the microscope 

 and screwed in or adapted the glasses which are intended 

 to be used, take a glass micrometer, and place it on the 

 stage in the same manner as if it were an object to be 

 viewed, then carefully adjust the focus of your instrument, 

 so that the lines on the micrometer may appear quite 

 sharp and distinct. Next, take a common ruler, or a slip 

 of card-board with equal divisions of some known mea- 

 surement drawn upon it, every tenth division being longer 

 than the rest, and fix it 20 inches from the eye, whilst 

 looking through the microscope ; then, whilst one eye is 

 directed to the rule or card-board, and the other to the 

 lines of the micrometer, seen in the microscope, ascertain 

 how many on the card are equal to a given number on the 

 micrometer. If the divisions on the latter be 1- 100th of 

 an inch, and one of them be equal to ten on the card, it 

 is clear that every division on the card will represent 



