GENERAL RULES FOR THE CONSTRUCTION OF STANDS. 121 



300 mm., so that with a table of ordinary height they can only be 

 used standing. The English and American instruments seem also 

 to be remarkable for large size. 



Large stands are sometimes arranged so that the body-tube turns 

 with the object-stage on a horizontal axis, and may, therefore, be 

 inclined at any desired angle ; a more convenient position of the 

 head is thus attained, and the body-tube may be used in a hori- 

 zontal position. These advantages, however, seem to us to be 

 more than counterbalanced by the inconvenience which the 

 inclination of the stage involves. Where a horizontal position 

 of the body-tube is required, which is seldom the case, the instru- 

 ment may be laid down. The horse-shoe stands are excellently 

 adapted for this purpose, the two ends of the horse-shoe forming, 

 with the stage, a heavy tripod. 



The desiderata of a good stand are, in onr opinion, (1) a sub- 

 stantial and convenient shaped brass-work ; (2) a firm and suffi- 

 ciently large stage ; (3) an illuminating mirror (plane and concave), 

 rotating on its centre and movable laterally ; and (4) a suitable 

 arrangement of diaphragm. We should estimate as of secondary 

 importance the movement of the mirror forwards and in a 

 vertical direction (easily arranged, indeed, in most instruments), 

 and especially the revolution of the stage round the optic axis. 

 These are properties which might be supplied in a simple manner ; 

 the former by the application of a condenser allowing of .oblique 

 illumination on all sides, the latter by the much less costly 

 revolving stage-plate. Where, however, the shape of the stand 

 admits of free movement of the mirror, this is preferable and 

 does not increase the expense. Finally, all other arrangements 

 which have been mentioned above are of a minor character, and 

 may be dispensed with in most researches. 



