330 



ACCESSORY APPARATUS 



designate it as we have done). It is a plano-convex lens of short 

 focus, two or three inches in diameter, mounted upon a separate 

 stand in such a manner as to permit of its being placed in a great 

 variety of positions. The mounting shown in fig. 277 is the usual 

 adopted in England ; the frame which carries the lens is borne 

 at the bottom upon a swivel joint, which allows it to be turned in 

 any azimuth ; whilst it may be inclined at any angle to the horizon, 

 by the revolution of 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 hori- 

 zontal arm may be lengthened or shortened; the lens may be 

 secured in any position (as its weight is apt to drag it down when 



FIG. 277A. 



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. 



A good form of the bull's-eye is made by Leitz, and is illustrated 

 fig. 277A. All the required movements are provided for, but in a 

 different way ; the clamping screws are by means of usual milled 

 heads. 



The plane side of the bull's-eye should be turned towards the 

 object. Some microscopists like to have their bull's-eye attached' to 

 some part of the microscope ; but if this is done, care must be taken 

 to attach it to a fixed part of the microscope, and not to either the 



