110 THEORY OF THE MICROSCOPE. 



the grounds already discussed, to apply these stops, &c., above the 

 illuminating apparatus. 



The so-called oblique illumination, which is now much employed 



for the resolution of difficult 

 details, consists of an incident 

 cone of light, whose axis is more 

 or less inclined to that of the 

 Microscope. Such a cone may 

 be obtained by placing the mirror 

 or the diaphragm somewhat later- 

 ally to the optic axis, or by cover- 

 ing up a portion of the diaphragm, 

 so that merely half, for instance, 

 of the original cone is effective 

 (Fig. 47). It is evident that if 

 FIG. 47. the angle of aperture of the dia- 



phragm were equal to that of the 



objective, and a deviation did not take place in the field of view, 

 every possible obliquity of illumination could be obtained by the 

 latter method. Since, however, the first condition is rarely fulfilled 

 (the diaphragm having almost always a relatively smaller aperture), 

 and the last cannot be fulfilled, it is in many cases advantageous 

 for the illuminating apparatus itself to admit of revolution round 

 a horizontal axis, thus furnishing incident light of any desired 

 inclination. Nevertheless, we do not estimate highly the value of 

 such contrivances. A diaphragm with lateral motion, such as 

 most modern Microscopes possess, combined with a suitable 

 adjustment of the mirror, will be quite as serviceable in most cases. 

 Among the really useful contrivances for illumination (especially 

 for the testing of objectives, &c.), that recently described by 

 Abbe (" Archiv fur mikr. Anatomic," Bd. ix. p. 469) deserves to be 

 recommended. It consists of two non-achromatic lenses, mounted 

 in a short tube. The upper lens is plano-convex, and is greater 

 than a hemisphere ; when properly adjusted, its plane-surface lies 

 slightly below the upper plane of the stage. The focus of the 

 combination is only a few millimetres above the plane-surface of 

 the upper lens, and is therefore very near to the object. The angle 

 of aperture of the emergent rays is 120 in water, so that the 

 marginal rays are inclined to the axis at an angle of nearly 60 in 

 water. 



