ACCESSORY APPARATUS. 



115 



it down when it is inclined, unless the tubes may be made to work, the 

 one into the other, more stiffly than is convenient) by means of a tight- 

 ening collar milled at its edges; and finally the horizontal arm is attached 

 to a sprung socket, which slides up and down upon a vertical stem. The 

 optical effect of such a ' bull's-eye ' differs according to the side of it 

 turned towards the light, and the condition of the rays which fall 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 surface should be turned towards the object; 

 and the same position should be given to it when it is used for procuring 

 converging rays from a lamp, this being placed four or five times farther 



Era. 87* 



Condensing Lens. 



Bull's-eye Condenser. 



off on one side than the object is on the other. But it may also be em- 

 ployed for the purpose of reducing the diverging rays of the Lamp to par- 

 allelism, for use either with the Paraboloid ( 105) or with the Parabolic 

 speculum to be presently described; and the plane side is then to be 

 turned towards the lamp, which must be placed at such a distance from 

 the ' bull's-eye/ that the rays which have passed through the latter shall 

 form a luminous 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 con- 

 centration of the rays already brought into convergence by the ' bull's- 

 eye.' An ingenious and effective mode of using the ' bull's-eye' con- 

 denser, for the illumination of very minute objects under high-power 



