Chapter VIII 



THE COMPOUND MICROSCOPE; THE ELECTRON MICROSCOPE 



THE COMPOUND MICROSCOPE 



In the chapter on the biophysical aspects of the eye the size of the 

 visual receptors in the retina of the human eye were discussed, and it 

 was shown that, in the region of greatest visual acuity, which lies at the 

 fovea centralis of the macula lutea, only very slender cones are found. 

 These cones are hexagonally packed with centers about 3 microns 

 apart. The retinal image must of necessity be at least 3 microns long 

 in order to excite two cones, from which we concluded that this distance 

 subtended an angle of 1 minute of arc, whose center of curvature was 

 a nodal point of the lens. The object producing the image must, 

 therefore, also subtend an angle of 1 minute of arc no matter how great 

 the visual distance. In a like manner any two details must subtend at 

 least an arc of 1 minute for them to be just separable. 



Resolving Power of the Eye 



The eye, at the distance of distinct vision (25 cm), can resolve two 

 points lying as close as 0.074 mm. At a distance of 1 meter from the eye 

 these two points may not be closer than 0.29 mm, for otherwise they 

 would fuse into one image. 



If the object is of constant size but at varying distances from the eye, 

 the visual angle in which the object is seen grows larger as the object 

 approaches the eye. Since the power of accommodation of the eye is 

 limited, there is a practical limit to which an object may approach an 

 eye and still maintain a focused image on the retina. By interposing a 

 simple converging lens between the eye and object the visual angle 

 subtended by the object is increased and therefore the angle subtended 

 by the retinal image is increased; this allows the eye to approach the 

 object very close and still retain the sharpness of its retinal image. The 

 lens aids the eye to produce a larger retinal image so as to make adjoin- 

 ing details spread out enough to fall on more than one retinal element, 

 thus making resolution possible. 



The microscopist uses a train of lenses to aid him to produce a high 

 resolution. He is not essentially interested in how big he can make his 



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