SPOT LENSPARABOLOID. 



tion is the spot lens : it is of very simple construction, 

 being a plano-convex lens, upon the upper flat surface of 

 which is drawn a circular spot of black varnish, so as to 

 exclude all central rays and prevent them passing into the 

 objective. It is shown in Fig. 57. 



FIG. 57. 



FIG. 58. 



In the best instruments the paraboloid is the dark- 

 ground illuminator, as rays of greater obliquity can be 

 obtained by the use of this accessory than by the spot lens. 

 It is shown in Fig. 58. 



In using the paraboloid the plane or flat mirror must 

 be used to throw parallel rays upwards, and these are 

 reflected from the internal 

 surfaces of the glass at such 

 an angle that with objectives 

 of moderate aperture the 

 field appears quite dark, the 

 rays simply illuminating the 

 object but not entering the 

 object glass. The track of 

 these rays may be seen on 

 reference to Fig. 59. 



A central stop or dark 



well is placed in the hollow 



? FIG. 59. 



of the paraboloid, and is 



capable of motion either upwards or downwards by means 



of a pin which runs through a hole drilled in the glass ; 



