THE BACTERIOLOGICAL MICROSCOPE. 



67 



of a microscope are principally convex, and the imperfections which 

 result must, if possible, be entirely overcome. These imperfections 

 are spherical and chromatic aberration. 



Spherical aberration results from the unequal refraction of 

 rays passing through lenses with equal curvatures. The rays passing 

 through an ordinary convex lens do not all come to the same focus. 

 The rays passing through the marginal portion come to a focus at a 



JPJ 



FIG. 12. SPHERICAL ABERRATION. 



point much nearer to the lens than the focus of the rays passing 

 through the more central portion of the lens (Fig. 1 2)* If the whole 

 aperture of the lens is used there must of necessity be blurring, for 

 at the point at which .the marginal rays form a distinct image the 

 central rays will be out of focus, and at the point at which the 

 central rays form a distinct image the marginal rays will have 

 diverged, causing indistinctness. 



This is partially remedied by using a dia*phragm 

 and shutting out the marginal rays ; but this is 

 at the cost of loss of light and diminution of the 

 angle of aperture. The difficulty is approxiinately 

 overcome in practice by using a combination of 

 lenses. The aberration of a convex lens is the 

 opposite of that of a concave lens (Fig. 13). The 

 makers of the best lenses endeavour to obtain this 

 correction as perfect as possible to get the sharpness 

 of the image, so essential in studying the mor- 

 phology of bacteria. 



Chromatic aberration is the result of the 

 unequal refrangibility of the coloured rays which 

 compose white light. If parallel rays of light pass through a 

 convex lens the violet rays, which are the most refrangible, will 

 come to a focus at a point much nearer to the lens than the 

 focus of the red rays, which are the least refrangible ; and the 

 intermediate rays of the spectrum will be focussed at points between 

 the red and the violet. A screen held at either of these foci 

 will show an image with prismatic fringes (Fig. 14). 



FIG. 13. COM- 

 BINATION o F 

 LENSES IN 

 ABBE'S HOMO- 

 GENEOUS IM - 

 MERSION. 



