ILLUMINATION BY KEFLECTED LIGHT. 



Ill 



It is now clear why for the production of the different kinds of 

 illumination we need only a sliding diaphragm of suitable aperture, 

 placed in the axis for central illumination, and more or less 

 laterally for oblique illumination. The circular movement in 

 Abbe's apparatus is effected by a rack and pinion ; for general 

 work, however, this mechanism is unnecessary, as the movement 

 of the diaphragm can be effected sufficiently well by the hand. 

 For further information we must refer our readers to Abbe's paper 

 (quoted above), and will only add that the apparatus is manu- 

 factured by Carl Zeiss, of Jena. 



2. ILLUMINATION BY KEFLECTED LIGHT. 



The illumination of opaque objects has the advantage, that micro- 

 scopic observation thus approximates to vision with the naked eye, 

 for the final image on the retina is produced in both cases by rays 

 which are reflected by the surface of the object. They are not, 

 however, strictly identical ; in microscopic vision, in consequence of 

 the greater aperture of the effective cone of light from the object, 

 the light and shadow are always distributed, cceteris paribus, some- 

 what differently. With this difference in the angle of aperture is 

 also connected the difficulty of providing an incident cone corre- 

 sponding with every cone that 

 reaches the objective from the 

 object-point, i.e., of so regula- 

 ting the illumination, that if 

 the pencils are produced back- 

 wards, every ray meets the 

 source of light. If, for instance, 

 A B (Fig. 48) is the upper 

 surface of a body with hemi- 

 spherical prominences, and g li 

 the effective portion of the 

 objective, the cone g Ji p is 

 reflected at p, so that its marginal rays take the directions 

 p k and p L With the dimensions given in the Fig., these rays 

 just touch on one side the surface of the object, and on the 

 other the margin of the objective, and thus reach the unlimited 

 source of light, the bright sky, through the window of the labora- 

 tory. Every obstruction to the incidence of the light may be 



FIG. 48. 



