CHAP. III.] 



THE MICROSCOPE. 



239 



The ordinary lens and the simple microscope have clone great 

 service to the sciences. The latter is 

 especially used for the preparation and 

 dissection of objects principally in 

 vegetable anatomy, for histologists pre- 

 fer the compound microscope for the 

 dissection of animal tissues. In this 

 case the magnifying power rarely ex- 

 ceeds 60 times, because, with more 

 powerful magnifications, the focus of 

 the lens is so short that there is no 

 room for manipulation. For simple 

 observations doublets may be used, 

 which magnify 500 times ; but, in 

 this instance, the focus of the magnify ing-glass is not the half of a 

 millimetre from the object. 



FIG. 176. Compound microscope. 



III. THE COMPOUND MICROSCOPE. 



In the compound microscope there are two systems of lenses ; the 

 one called the eye-piece, because it is placed nearest to the eye ; the 

 other, the object-glass, because it is turned towards the object which 

 is to be magnified. In the most simple and rudimentary instru- 

 ments the object-glass is a bi-convex lens, which furnishes an 

 already magnified but reversed image of the object. It is this image 

 which is examined by the eye-piece, which therefore acts as a 

 magnifying-glass ; with this exception, that the magnifying-glass 

 magnifies the image and no longer the object. 



Fig. 177 shows the path of the luminous rays in such a compound 

 microscope. 0' is the eye-piece, and the object-glass, in front of 

 which is seen the little object la. The object-glass produces an 

 enlarged image at the focus of the eye-piece. This image, which 

 in turn serves as an object to the .eye-piece, is reversed, and, as 

 the eye-piece only magnifies it without correcting it, the eye sees 

 the object reversed, as if it were at AB that is, at the distance of 

 distinct vision. 



Such is the optical apparatus of the compound microscope, 

 reduced to the most simple statement, for the sake of making the 



