122 



APPENDAGES TO THE MICROSCOPE. 



upon it. The position of least Spherical Aberration is when its 

 convex side is turned towards parallel or towards the least diverging 

 rays ; consequently, when used by Daylight, its plane side should 



be turned towards 

 FlG - 70 - the object; and 



the same position 

 should be given to 

 it when it is used 

 for procuring con- 

 verging rays from 

 a Lamp, the lamp 

 being placed four 

 or five times far- 

 ther off on one 

 side than the ob- 

 ject is on the other. 

 But it may also be 

 employed for the 

 purpose of reduc- 

 ing the diverging 

 rays of the Lamp 

 to parallelism, for 

 use either with 

 the Parabolic illu- 

 minator (§ 85), or 

 with the Side 

 E eflector to be pre- 

 sently described} 

 and the plane 

 side is then to be 

 turned towards 

 the Lamp, which 

 must be placed 

 at such a distance 

 from the Conden- 

 ser that the rays 

 which have passed 

 through the latter 

 shall form a lumi- 

 nous circle equal 

 to it in size, at 

 whatever distance from the lens the screen may be held. For viewing 

 minute objects under high powers, the smaller Condensing Lens 

 may be used to obtain a further concentration of the rays already 

 brought into convergence by the 'Bull's Eye' (§ 123). 



91. The Illumination of Opaque objects may be effected 

 by reflexion as well as by refraction ; and the most convenient 

 as well as most efficient instrument yet devised for this purpose is 

 the Parabolic Speculum of Mr. R, Beck (Fig. 71), which is attached 



Bull's-Eye Condenser. 



