190 THEORY OF MICROSCOPIC OBSERVATION. 



different from that of ordinary vision. Through the Microscope we 

 see the objects, as a rule, by transmitted light ; with the naked 

 eye, by reflected light. Any given point of the Microscope image 

 therefore appears the brighter the greater the number of the rays 

 which before their entrance into the objective, if produced back- 

 wards, are directed towards the corresponding point of the object. 

 If these rays form a perfect cone of light whose base fully occupies 

 the optically effective part of the anterior surface of the objective, 

 the point appears of maximum brightness. If the base of the cone 

 is only half as large, the point appears in the image only half 

 as luminous and so on. The Microscope forms therefore, to a 

 certain extent, a shadow-image, in which the opaque parts of the 

 object appear dark, and the transparent ones more or less bright, 

 provided the inequalities and differences in density of the object, in 

 consequence of the refraction they produce, contribute essentially 

 to the distribution of light in the image. 



This obtains also in vision with the naked eye, if transparent 

 bodies are observed under the same conditions. Nevertheless, the 

 image which we see in this case is essentially different from that 

 formed in observation through the Microscope; and contrary 

 assertions which are met with here and there in micrographic 

 works 1 must be regarded as erroneous. They are based upon the 

 tacit assumption that the unequally large apertures of the incident 

 cones of light directed towards the eye by the Microscope have no 

 influence upon the resulting effect of light. A mathematical 

 discussion of this question proves that the distribution of light and 

 shade in the microscopic image is dependent upon the angle of 

 aperture of the instrument, and is not therefore the same even for 

 different amplifications, much less for the naked eye. 



There can be no doubt that objects are seen in the Microscope 

 differently than with the naked eye. The observer with the Micro- 

 scope begins by viewing images which are new and unusual to 

 him ; he must first learn to interpret them correctly, precisely as 

 with the words of a foreign language. Although practice is most 

 essential and cannot be replaced by theory, still a theoretical 

 foundation of microscopic vision would not be superfluous even to 

 the experienced observer, since it presents, in many cases, useful 

 data for further deductions. Hence we have submitted a series 



1 Cf. for instance, Harting, "Das Mikroskop," 1st ed. p. 339; 2nd ed. ii. 

 p. 26. 



