218 



THEORY OF MICROSCOPIC OBSERVATION. 



FIG. 118. 



medium plane of adjustment, and often as distinctly as with a 

 somewhat flattened air-bubble. 



The formation of these images is 

 evident from Fig. 118. The edge of 

 the aperture or depression causes on 

 the one side a deviation of the rays 

 emanating from a distant point 

 (therefore incident nearly parallel) 

 towards n, and on the other side a 

 deviation by total reflexion towards 

 m. Both the pencils, after their pas- 

 sage through the object, appear to 

 proceed from a small space 0, in 

 which they intersect when pro- 

 duced backwards ; they therefore 

 form in o the virtual image of the 

 distant source of light. If the latter 

 is moved further to the right, its 

 image also moves in the same direc- 

 tion, as obtains with virtual images. 

 The formation of the image is not dependent upon the regu- 

 larity of the refracting surfaces, as has been assumed in our 

 figure. It is unimportant whether the refracted rays are exactly 

 parallel, or whether they diverge or converge slightly, since the 

 space in which their productions intersect becomes thereby some- 

 what larger or smaller. It always remains so small that, com- 

 pared with the deviations which the cover-glass produces, it may 

 be neglected, and to the observing eye will appear as a point. 



Though the incident pencils reach the eye through one edge 

 only of the aperture i.e., do not intersect if produced backwards 

 an indistinct image is seen precisely as with rays which pass 

 through a minute aperture. It does not, however, in this case 

 assume any definite position, but appears the greater the lower the 

 focal adjustment With favourable adjustment even those rays 

 which traverse the aperture without refraction must contribute to 

 the formation of this image. 



In whatever manner the image is formed, whether it appears 

 more or less indistinct or sharply outlined, the focal adjustment to 

 a higher or lower level will always produce phenomena similar 

 to those observed in the virtual image of the air-bubble. On 



