AND THE MICROSCOPE. 29 



is in no way different from it. In the compound micro- 

 scope, an apparatus of this kind is made use of in such 

 a manner, that the enlarged picture of the object is not 

 seen immediately by the eye, but looked at through a 

 simple microscope and thus again considerably magnified. 

 For example, if the picture is a hundred times larger than 

 the object, and we magnify this picture ten times, the 

 object, of course, appears a thousand times larger. There- 

 fore, the compound microscope is composed of a double 

 optical apparatus ; first, the glasses which are directed to 

 the thing to be examined, or object, of which they project 

 an enlarged picture, these are called the object-glasses, or 

 objectives ; secondly, of a simple microscope, by which the 

 enlarged picture of the object is again magnified, and which 

 from being turned towards the eye, is called the eye-piece 

 or ocular. 



From the foregoing statements, it might be imagined 

 that we can magnify to any degree we like, since the size 

 of the picture depends solely upon the extent to which we 

 approach the object towards the object-glass, and then the 

 enlargement of the picture is only conditional on the approxi- 

 mation of the picture to the eye. But so many practical 

 difficulties oppose this theoretical possibility, that the instru- 

 ments actually constructed all fall far short of the theore- 

 tical limit. 



I shall here mention only the most important circum- 

 stance, and in order to make this clear, make use of a very 

 familiar fact. Books intended for very general use, such as 

 bibles and hymn-books, are printed in many different types, 

 sometimes with very small, sometimes moderate sized and, 

 for the old and weak-sighted, with very large letters. A 

 single word in the type of the last kind, is perhaps six 

 times as large as in the first and is very easily recognized, 



