MICRO-ORGANISMS AND FERMENTATION. 



The lenses form the most important part of the microscope ; the system 

 turned towards the eye is called the eyepiece, and that turned towards the object 

 is called the objective. 



When a bundle of parallel rays of light strikes a convex lens, the rays are 

 refracted and collect at a point on the other side of the lens, called the f OCUS. 

 The distance between this point and the lens is called the focal distance. If 

 a small object is placed on the stage at a slight distance beyond the focal length 

 of the lens and illuminated by the mirror, the rays passing through the lenses 

 of the objective will not be parallel, but will diverge, and so form a magnified 

 image of the object. The smaller the focal length, the greater the magnification. 

 This real inverted and magnified image formed by the objective, which must fall 

 exactly in the focus of the eyepiece, is seen through the latter as an imaginary, 



magnified image of the picture formed by the 

 objective. The whole magnification secured by 

 the microscope is, therefore, a product of the 

 magnification due to the objective and that due 

 to the eyepiece. The magnification is always 

 expressed as a linear and not as a quadratic 

 dimension i.e., the relationship between the 

 length of a line as seen through the microscope 

 and its length as seen by the naked eye. 



From any given point of the object on the 

 stage of the microscope a bundle of rays may 

 pass through the lens. The angle which the 

 outermost rays of the bundle form is called the 

 angular aperture. It is not customary to 

 allow all the rays of the angiilar aperture to 

 pass through the system of lenses which make 

 up the objective. A larger or smaller number 

 of the peripheral rays are excluded by means of 

 diaphragms consisting of metal discs with round 

 apertures, which are of various sizes, and fit into 

 the opening on the stage. In this way the 

 Fi - 2 - actual angular aperture of the objective is 



reduced. The peripheral rays would give an indistinct picture, but the picture 

 would, on the other hand, lose in clearness if too many of the outer rays were 

 excluded. 



For this reason the objective is so constructed that the aperture shall be 

 as great as possible. An expression for the value of the latter is given by the 

 numerical aperture (the sine of half the angular aperture multiplied by the 

 index of refraction of the medium in front of the lens ah*, water, oil, etc.). 



When the rays of light are refracted in the objective some of them are immedi- 

 ately split up into the component coloured rays, and the image shows coloured 

 edges (chromatic aberration). To avoid this the objective is made up of 

 several different lenses prepared from various kinds of glass (crown glass, flint 

 glass) which possess different refractive powers. It is thus possible to prevent 

 any indistinctness of outline. 



