2 PROGRESS IN MICROSCOPY 



parison is to be proceeded with, by reverting to the problem as con- 

 sidered under its initial aspect. Since the structure of images of typical 

 objects such as disks, lines, rectangles, edges and so forth, is known 

 beforehand, the geometry of the detail examined can be determined 

 by comparison. This applies not only to specimens whose details are 

 featured by differences in brightness but also to thoroughly transparent 

 specimens in which details consist solely in refraction-index and thick- 

 ness differences. Therefore, more often than not, it is by close exami- 

 nation and comparison of the images, together with the theoretical 

 results obtained, that the problem of image interpretation may be solved. 



2. IMAGE OF A LUMINOUS POINT EXHIBITED BY THE OBJECTIVE 

 OF A MICROSCOPE IN MONOCHROMATIC LIGHT 



Figure 1.1 shows a microscope diagrammatically, Oi being the 

 objective and O.^ the eyepiece. Oi is, for instance, the first front lens 

 of the objective. The specimen P emits light only through a small 



Fig. 1.1. Image of a luminous point produced by a microscope objective. 



aperture whose size is far smaller than the resolving power of the 

 objective and plays the part of the object. Assuming the light emitted 

 by A to be monochromatic, // is the refraction index of the object- 

 medium and u the angle under which the light-ray of the first objective 

 lens is seen from A. 



The objective O^ (numerical aperture (N. A.) u sin u) shows in A^ an 

 image of A. Required: to know the structure of the image A'q, i.e. the 

 distribution of light-intensity in the plane .t, across A'^. 



The sphere Zq (centre at ^4), is a wave surface in the object space. 

 Assuming the objective Oi to be perfect, wave surfaces in image space 

 are spheres such as A',. The centres of these waves are at point ^4,',, 

 which is a geometrical image of A. According to Huyghens' principle, 

 the image A'^ is formed as if the various points of the wave surface 

 i7- were actual sources of light in the same vibratory state. Any point. 



