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 be 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. 



