153 



The qualification of the convergence is necessary, since 

 in the use of convergent pencils of large angles we are 

 pro rata departing from direct and verging upon oblique 

 illumination. Assuming, then, the correctness of what both 

 theory and practice teaches, the niicroscopist will choose for 

 central or direct illumination a convergent pencil of rays of, 

 however, not too great an angle, and may on option stoj) out, 

 and probably with good effect, the central or rays parallel 

 with the optical axis of the instrument. The focal point of 

 the illuminating cone, however, is not that which the object, 

 as at first sight would seem natural, should occupy. Careful 

 observation will show that when the object is situated some- 

 what within the focus of the illuminating pencil, the best 

 result is obtained, and this separation or distance between 

 the object and focus of the illuminating pencil increases in 

 inverse ratio with the power of the lens. 



Next as to the most suitable means for eifecting such 

 illumination. Universally almost, it may be said, the concave 

 mirror is that M'hich is furnished by the maker for such pur- 

 pose. The use of this, however, is objectionable, by reason 

 of the double reflection always attendant ujDon the use of 

 any mirrored surface, of no matter what form, viz., the 

 primary or greater reflection from the silvered surface of the 

 back, and the secondary or lesser reflection from the surface 

 of the glass itself, this double reflection causing two sets of 

 rays to enter the objective, in addition to those emanating 

 from the object, which militate greatly against the sharpness 

 of definition observable on a more proper illumination. Of 

 this one can easily satisfy oneself, more especially when 

 working with high powers. The various achromatic con- 

 densers, kettle-drum illuminators, and other addenda, con- 

 densing light at a large angle upon objects, do not here claim 

 consideration. In the use of such accessories we are not 

 employing central or direct illumination ; but, on the con- 

 trary, oblique or eccentric — a mode of illumination which 

 subsequently will claim our attention. It may, perhaps, be as 

 well, however, here to mention that such oblique illumination 

 differs from oblique illumination as usually understood, in 

 that it is omni-lateral, or, in other words, oblique from every 

 side, whereas, in general, when speaking of an object as 

 viewed by oblique light, unilateral, or light falling upon the 

 object from one side, is meant. 



The means or instrument best adapted to its accomplish- 

 ment is, without doubt, the achromatic lenticular prism of 

 Abrahams, or that of Messrs. Powell and Lealand. This 

 consists of a rectangular prism of flint glass, with one of its 



