>;ITX 



PART IV. 



THEOKY OF MICEOSCOPIC OBSEBYATI01S T . 



MICROSCOPIC observation differs in many respects from ordinary 

 observation with the naked eye. It demands, therefore, long 

 practice, in order to arrive at that certainty of judgment in regard 

 to the sensations received from the light which alone assures the 

 correctness of the observation. To see through the Microscope, and 

 interpret the form and nature of an object in accordance with 

 what has been thus seen, is an art requiring special training. 



Amongst the peculiarities of microscopic vision we may 

 primarily remark, that the image which we observe does not 

 exactly correspond to the object itself, but only to a particular 

 sectional plane to which we happen to adjust the focus. Whatever 

 lies above or below is indistinct to the eye, or invisible. The 

 Microscope therefore gives us direct information only of the 

 dimensional relations of the different sectional planes, but not of 

 their distances in the direction of the axis of the instrument. The 

 latter can in many cases be approximately estimated ; for instance, 

 we may distinguish at once whether a given object is spherical, 

 flattened, or compressed at its sides, but a more accurate deter- 

 mination of its form can only be attained by turning it on its axis 

 that is, by the combination of different views. 



This is not so in ordinary vision. A mere glance is in this case 

 often sufficient to inform us of the form and grouping of closely 

 adjacent bodies, because the limits within which the eye sees dis- 

 tinctly at one view lie somewhat distant from each other. We 

 therefore see the bodies in their stereometric form, and in their 

 positions relative to those which are contiguous to them. Whether 

 we observe with two eyes or one is immaterial, precisely as with 

 the Microscope. 



Secondly, the illumination in microscopic observation is entirely 



