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INTRODUCTION. 



I. USE OF THE MICROSCOPE AND EXAMINATION OF 



MICROSCOPIC OBJECTS. 



The Microscope as a means of investigation might perhaps be thus defined : the microscope 

 is an optical instrument constructed in order to enable us to investigate the characters 

 and properties of those objects which we are unable to study with the unassisted eye, on 

 account of their minute size. 



In using the microscope, exactly the same precautions must be adopted and just the 

 same course pursued, as when the object under examination is distinctly visible to the 

 unaided eye. We have thought it requisite to introduce this formal definition of the true 

 use of this valuable instrument, from a knowledge of the fact, that in this country at least, 

 the microscope has hitherto been used chiefly as a means of viewing minute objects, and 

 judging of their natm'e from the simple inspection of them under the conditions in which 

 they naturally or accidentally occur. Such means, the most careless observer must be well 

 aware, is never trusted alone in the examination of objects visible to the naked eye, being 

 almost sure to lead to erroneous conclusions. Consider the common course pursued in the 

 examination with the unaided sight, of a body for the first time presented to our notice ! 

 The first point is the examination of its general appearance and colour ; the relative position 

 of the eye of the observer and the object is then changed, so that an idea of its solidity may 

 be obtained ; its weight is next perhaps determined by taking it in the hand ; it is presented 

 to the light in various ways, in order to judge of its transparency, and of the optical pro- 

 perties of its surface. If the object be at a distance, its size is judged of by comparing its 

 apparent size with that of adjacent bodies, whose dimensions are approximatively known ; 

 and its luminousness is also taken into consideration, it being know n generally that the 

 nearer bodies of the same size are to us, the more luminous they appear. The observer 

 then is either satisfied with the conclusions drawn from reasoning upon the results thus 

 obtained, or he makes besides a chemical examination. When the microscope is called into 

 use, many of these modes of determination of the properties and nature of objects are still 

 applicable, yet mostly is the observer satisfied with simple inspection ; the object is then 

 perhaps " put up," and exhibited to an astonished multitude, probably as many opinions 

 being formed of its nature as there are observers present, and possibly all of these incorrect. 



Another frequent source of erroneous judgment consists in forming an opinion upon the 



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