186 THE MICROSCOPE. 



immersion condenser of Powell and Lealand is an im- 

 proved form, consisting of the truncation of the vertex 

 of the upper lens of the condenser, and admits of 

 the lower lens being brought into closer proximity, 

 when the marginal rays become more effective. Its 

 speciality is the conversion of axial light into con- 

 densed obliquely incident light by the refraction of 

 the condenser. 



For the illumination of opaque objects under high 

 powers, Tolles of Boston, U.S.A., introduced a verti- 

 cal illuminator into the body of the microscope close 

 to and above the objective. 



The Vertical Illuminator consists of a small silver 

 speculum (Professor Smith), or a movable disc of thin 

 glass (Beck), or a small piece of parallel glass, placed 

 at an angle of 45 (Powell and Lealand), and fixed in a 

 short tube, with a side aperture, interposed between 

 the objective and the body of the microscope ; by 

 which means a pencil of light entering at the aperture, 

 and striking against the speculum or inclined surface 

 of the disc, is reflected downwards through the objec- 

 tive and upon the object placed on the stage of the 

 microscope. The object-glass is thus made its own 

 achromatic condenser. When this form of illuminator 

 was introduced, it was soon discarded on account of the 

 halo or fog which surrounded the image, and which 

 was caused, as Mr. Stephenson explained, by the reflec- 

 tion, at the upper surface of the cover-glass, of the 

 rays transmitted through the objective. With the 

 introduction of the oil-immersion objective all this 

 fogging disappeared ; the front lens of the objective, 

 the intervening stratum of oil, and the cover-glass of 

 the object all become optically continuous, so that the 

 upper surface of the cover-glass virtually ceases to 

 exist, the only reflection being from its under surface, 

 when dry objects are used. " The explanation is that 

 if the vertical illuminator be adjusted, and used with 

 an immersion objective, having a numerical aperture 

 greater than I'O, focussed on a plane glass slip, it is 

 evident that (practically) all that part of the pencil 

 comprised within the numerical aperture I'O will 



