144 



ACCESSORY APPARATUS. 



the horizontal tube to which it is attached, around the other 

 horizontal tube which projects from the stem ; by the sliding of 

 one of these tubes within the other, again, the horizontal arm may 

 be lengthened or shortened ; the lens may be secured in any posi- 

 tion (as its weight is apt to drag it down when it is inclined, unless 

 the tubes be made to work, the one into the other, more stiffly 

 than is convenient) by means of a tightening collar milled at its 

 edges ; and finally the horizontal arm is attached to a spring 

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

 optical effect of such a lens 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 side 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, the 

 lamp being placed four or five times farther off on one side, than 

 the object is on the other. But it may also be employed for the pur- 

 pose of reducing the diverging rays of the lamp to parallelism, for 

 use either with the parabolic illuminator ( 61), or with the side- 

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

 be turned towards the lamp, which must be placed at such a dis- 

 tance from the condenser, that the rays which have passed through 



the latter shall form a lumin- 

 ous circle equal to it in size, 

 at whatever distance from the 

 lens the screen may be held. 

 Even where the large " bull's- 

 eye" condenser is provided, it 

 is well to have a smaller con- 

 densing lens in addition ; and 

 this, which is usually a double- 

 convex lens, may either be 

 mounted on a separate base 

 (Fig. 45), or may be attached 

 to some part of the micro- 

 scope. (In Messrs. Smith and 

 Beck's large microscope, Fig. 

 29, two sockets with binding- 

 screws, one for the condensing 

 lens, the other for the side-re- 

 flector, are seen in the "limb.") 

 This condensing lens is suffi- 

 Condensing Lens. cient by itself for most ordi- 



nary purposes; and it may 

 also be used to obtain a greater concentration of the rays already 

 brought into convergence by the bull's-eye ( 93). 



65. The illumination of opaque objects may be effected by re- 

 flection, as well as by refraction ; and a very advantageous means 



FIG. 45. 



