32 Modern Microscopy. 



convenience, as by this means light from the lamp can 

 come direct through the sub- stage condenser for critical 

 work. A few years since it was a rule in manipulation 

 to obtain oblique light by this arrangement for the ex- 

 hibition of stride on finely-marked diatoms, etc. ; also the 

 sub-stage itself could be swung on an arc, having its 

 centre on the upper surface of the stage in the optical 

 axis. In the sub-stage, various ^condensers were used at 

 the point giving the obliquest ray that the objective could 

 receive. The instrument on page 34 illustrates the arrange- 

 ment. This method is not now adopted in England, the 

 advent of the achromatic condenser having rendered it 

 superfluous; but our American friends still cling tena- 

 ciously to it, and advocate its use. The mirror, in this 

 form, can be brought above the stage for opaque illustra- 

 tion, and this element of advantage largely influences in its 

 favour. With a condenser in the sub-stage in the optical 

 axis, if oblique light be desired it can be obtained to the 

 nicest degree by means of stops, with slots cut in them, 

 placed beneath the optical combination. 



The Mirrors should be plane and concave, and hung in 

 a gymbal, giving universal movements, and have a means 

 of adjustment to focus in a vertical direction. The plane 

 mirror is always used with the condenser, spot lens, etc., 

 and with very low-power objectives, but the concave when 

 the condenser is not employed and the maximum amount 

 of light is desired. 



TESTING A MICROSCOPE. 



The following are some points to be specially examined 

 when purchasing a microscope : The motions should be 

 perfectly smooth, with no lumpy feeling, and there should 

 be no backlash. This latter can be detected best by hold- 

 ing gently the part that is being moved by the pinion, 

 and then attempting to rotate the pinion. If the pinion 



