CATOPTRIC ILLUMINATOR. 185 



will give a pencil of light at varying degrees of ob- 

 liquity throughout the range of the aperture of the 

 condenser. The azimuthal direction of the incident 

 pencil will be controlled either by rotating the object 

 or the condenser carrying the diaphragms ; whilst the 

 rotation of the spiral in the fixed slot will not change 

 the direction in the azimuth but in altitude, so far as 

 the aperture of the condenser will permit. 



Mr. J. W. Stephenson's " Catoptric Immersion Illumi- 

 nator " attains its object in a simple way. Fig. 119 

 represents the form and size of the little piece of 

 apparatus. It is a plano-convex 

 lens worked on a 1-inch tool, and 

 having a diameter of 1*2 inches, 

 which is then edged down to 1 

 inch, as being more convenient in 

 size, and as giving an aperture 

 sufficient for the purpose. The 

 upper or convex side of the lens is 

 cut down or flattened, so as to 

 give a surface of -A- of an inch in FIG. 119. catoptric im- 

 diameter, with which the slide is 

 to be brought into contact, by a drop of oil, glycerine, 

 or water. The upper curved surface is silvered ; be- 

 neath the lens a flat silvered plate gV of an inch thick, 

 and corresponding in size and position with the upper 

 flattened surface, is balsamed. The incident ray is 

 thus rendered normal to the under surface, and is 

 thrown back on the plane or under surface of the 

 lens, whence the more oblique rays falling beyond the 

 central angle are totally reflected and conveyed to a 

 focus. A stop is placed about an -Jth of an inch or less 

 below the condenser, and the opening used is of a lens- 

 shaped form, which admits a broad beam of light with- 

 out appreciable spherical aberration. The Iris dia- 

 phragm greatly improves this illuminator. 1 



The Oil Immersion Condenser. In this is combined 

 the latest improvement in immersion condensers. In 

 operation, an oil-medium possesses superior advantages 

 in connection with high-angled objectives. The oil 



(1) Journal ofR. M. S., Vol. II., p. 36, 1879. 



