THE STANDS OF MODERN OPTICIANS. 125 



in //. If the cross-bars I b, which with the tube and column 

 form the movable parallelogram, are of good construction and 

 properly fitted, the focusing movement will presumably be very 

 smooth and free from lateral play. 1 (2) An arrangement of the 

 stage which will, among other things, admit of extremely oblique 

 illumination, is very desirable. For this purpose the aperture 

 of the stage should be bevelled out beneath, otherwise the 

 obliquity of the rays will be limited by the margin. The substage,, 

 diaphragms, &c., must be removable, as arranged in many of the 

 larger instruments. With regard to the other appliances which 

 are mentioned in the catalogues, we fail to find, almost without 

 exception, any contrivance which, in investigation with polarized 

 light will allow not only the crossed Nicols, but also the differently- 

 coloured selenite plates to be fixed, and the object alone rotated on 

 a vertical axis. 2 The revolving stage-plate answers this purpose 

 only when the necessary space for the reception of the selenite 

 plates is reserved between it and the fixed stage. 



With reference to the adjustment of the mirror, the older 

 arrangements for this purpose are well known. In recent times 

 Nachet, Zeiss, and others have applied a movement forwards, as 

 well as from right to left an improvement which in certain cases 

 much facilitates oblique illumination. 



The stands of German and French opticians have reached such 

 a degree of perfection that they completely satisfy ordinary 

 requirements. For unusual investigations, however for instance, 

 in physical researches some additional appliances are sure to be 

 necessary. 



1 The two cross-bars b b, on each side, are attached to the column by two 

 screws (shown in dotted lines) permitting pivot motion in the vertical, 

 without lateral play ; the socket of the body- tube (joined to //) is held at 

 the front ends of the cross-bars between the points of four screws (shown in 

 dotted lines) : the friction is therefore confined to the motion of the cross-bars 

 between the eight screws (i.e., four on each side). When the focusing- screw 

 s is turned, the solid bar //is pushed upwards, or the spiral spring shown 

 above it presses it downwards, and the cross-bars assume a diagonal position 

 similar to the motion of an ordinary parallel ruler, of which one side is 

 stationary ; // therefore moves slightly backwards, from the normal position 

 figured, carrying with it the optic axis. This motion of the optic axis presents 

 difficulties of centering in combination with a rotating stage. [ED.] 



2 This movement is applied to the best English and American stands. [ED.] 



