112 , . SELECTION AND USE 



it, the latter may now be adjusted exactly over the centre of 

 rotation of the stage. With common objectives this answers 

 very well, but first-class objectives are apt to show the effect 

 of being thrown out of centre. 



Where rotation in regard to the illumination alone is required, 

 several plans may be employed. One of the oldest is that 

 found in the large microscopes of Hartnack and others, and 

 recently adopted by Mr. Browning. This device consists in 

 forming the stage in two pieces, the lower part being firmly 

 connected with the foot, and the means of illumination, while 

 the upper part, which rotates on the lower, is rigidly attached to 

 the arm which carries the body. In this way the body, the arm, 

 the upper part of the stage and the object may all be simultane- 

 ously rotated in relation to the illumination, and for ordinary 

 purposes this answers very well. 



When polarized light is used, however, it is obvious that it 

 is impossible to rotate the object in relation to the polarized 

 ray, without also rotating the analyzer, and, as every one that 

 has worked much in this direction knows, it is often of great 

 importance that both polarizer and analyzer should be kept 

 stationary, while the object itself rotates between them. 



The little diatom stage devised by the author, and shown in 

 Figure 26, page 114, also affords very simple and convenient 

 means for rotating objects either above or below the stage. 

 And it would be a very easy matter to so construct it, that its 

 centre of rotation might be accurately adjusted to the optic 

 axis of the instrument. 



Hot Stage. It is frequently desirable to keep certain pre- 

 parations at a given temperature or to raise them considerably 

 above the usual temperature of the atmosphere, so as to observe 

 the ac ion of reagents upon them and the effect of heat upon 

 their vitality, where living organisms are under observation. 

 Various devices have been worked out for this purpose. A 

 stage consisting of two plates, each with a central aperture, 

 and so united as to form a tight box through which a heated 

 fluid is made to circulate, is probably the best. For tempera- 

 tures under 212 F. water is the best liquid; for higher tem- 

 peratures oil or saline solutions may be used. 



