THE MICROSCOPE. 
distance from c greater than half the radius, and an inverted 
image of it is formed at o o, which, as in the case of the refracting 
microscope, is looked at with an eye-glass e e. 
The great improvements which have taken place within the last 
twenty years in the formation of the object-glasses of refracting 
microscopes, have rendered these so very superior to reflecting 
Fig. 3. 
microscopes, that the latter class of instruments having fallen so 
completely into disuse, it will not he necessary here to notice 
them further. 
In what has been explained, the general principle only of the 
microscope has been developed ; many important circumstances of 
detail upon which its efficiency mainly depends must now be 
noticed. 
8. The conditions which render the vision of an object with the 
microscope clear and distinct are essentially the same as those 
which determine the clearness and distinctness of our perception 
of an object with the naked eye. It will be found, by reference 
to our Tract upon “ the Eye,” that these conditions are three :— 
1. That the visual angle should be sufficiently large; 
2. That the outlines and lineaments of the object should be 
sufficiently distinct; and 
3. That the object should be sufficiently illuminated. 
It is evident that if any one of these conditions fail to be ful¬ 
filled, our visual perception of the object will be defective. If 
the object, for example, be exceedingly minute, though it be 
perfectly delineated and strongly illuminated, it will be either 
altogether invisible, or will appear as a mere speck. 
If its outlines and lineaments be ill-defined, as when a tree or 
other object is seen through a mist, our perception of it will also 
be defective ; and in fine, though it have sufficient magnitude and 
be perfectly delineated, we may fail to see it distinctly for want 
of sufficient light upon it, as when we look at objects towards the 
close of twilight. 
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