MICROSCOPICAL PREPARATIONS. 3 



To secure greater magnification a system of immersion is used, the space 

 between the front lens of the objective, which is made of crown glass, and the 

 cover glass being filled with a strongly refractive medium, water or oil. The 

 immersion is homogeneous if, as is often the case, the oil has the same index of 

 refraction as the cover glass and the front lens. In this way an increase of the 

 numerical aperture is secured, and, therefore, a greater resolving power the 

 limit of definition for the smallest perceptible details. 



The eyepiece consists of two lenses, the upper, which comes into direct 

 contact with the eye, and the lower, called the collective lens, which collects 

 the rays of light so that the field Of vision is reduced, and is, therefore, more 

 easily surveyed. Between the two lenses a diaphragm is inserted in order to 

 further reduce the field. The collective lens is also of importance in securing, 

 along with the upper lens, the exclusion of the coloured edges of the microscopical 

 picture. 



The greater the curvature of the lenses in the eyepiece, the more do they 

 magnify the image projected by the objective, which at the same time becomes 

 darker and less distinct. To obtain a well-lighted field when working with high 

 magnifications, it is necessary to use strong objectives and weak eyepieces. 



It is well known that the lenses of the human eye alter their shape according 

 to the distance of the objects that are under observation. They can accommodate 

 themselves, and, by projecting, shorten the focal length, and thereby produce 

 a sharp image on the retina. On the other hand, by a reduction of the convexity 

 the focal length is increased, and thus a clear image of distant objects may be 

 thrown on to the retina. If the eye is short-sighted or long-sighted, the tube 

 of the microscope must be set to suit the focal length of the eye, and the size of the 

 image will differ for abnormal eyes. The normal focal distance is considered 

 to be 250 mm., and tables Of magnification are based on this. The actual 

 magnification for any individual eye must be established by a special calculation 

 with the help of a glass micrometer. 



The illumination of the object is secured by a mirror placed below the 

 stage which is capable of movement in all directions, so that the object can receive 

 direct or oblique illumination, and so that the mirror can be fixed at different 

 distances from the object. For low powers the plane side of the mirror is used, 

 for high powers the concave side. With ordinary magnifications it is of import- 

 ance to secure suitable illumination, as the eye soon tires if the light is too strong. 

 Instead of the usual diaphragm in the stage, an iris diaphragm may be used, 

 enabling the aperture to be reduced or enlarged by means of a number of sickle- 

 shaped leaves sliding over each other. To give illumination over a large surface 

 (e.g., in the examination of coloured substances), a combination of lenses known 

 as a condenser is introduced between the mirror and the preparation. When 

 studying an object, the separate parts of which can only be distinguished by 

 differences in their refractivity, a narrow bundle of rays must be used, and this is 

 secured by placing a diaphragm with a smaller opening in the aperture of the stage. 



The microscopical examination of the organisms of fer- 

 mentation throws light upon their size, form, colour, the 



