BULL'S-EYE CONDENSER. 



183 



mode of viewing objects is often applied to semi-opaque, 

 or even some transparent ones, to demonstrate pec uliari ties 

 of structure. A condensing lens is used for converging 

 rays from a lamp upon the 

 mirror ; or for reducing the 

 diverging rays of the lamp 

 to parallelism, for use either 

 with the parabolic illumi- 

 nator, or Ross's side-reflector. 

 A plano-convex lens (fig. 

 106), of about three inches 

 focal length, is the form ge- 

 nerally adopted ; it is borne 

 upon a swivel-joint, which 

 allows of its being turned 

 in any direction, and placed 

 at any angle ; the tube is 

 double, and thus admits of 

 being lengthened or short- 

 ened. When used by day- 

 light, its plane side should 

 be turned towards the ob- 

 ject, and the same position 

 should be given when used 

 for converging the rays from 

 a lamp ; but when used 

 with the parabolic or side- 

 reflector, the plane side 

 must be turned towards the 

 lamp. Sometimes a smaller, 



double convex lens is made use of in addition ; this is 

 then often fixed into the stage of the microscope. 



In fig. 107 the bull's-eye lens, c, slides up and down 

 a brass rod, screwed into a solid foot ; and made to con- 

 centrate the light upon the object from the table gas- 

 lamp, d. Mr. Brooke's method of viewing opaque objects 

 under the highest powers of the microscope (the J and yW 

 inch object-glass) is effected by two reflections. The rays 

 from a lamp rendered parallel by a condensing lens are 

 received on an elliptic reflector, the end of which is cut 

 off a little beyond the focus; the rays of light converging 



Fig. 106. — The Condensing Lcr, 



