THE PROJECTION MICROSCOPE 20! 



brightly on the screen, he has fairly grasped the powers of the 

 instrument with the oxy-hydrogen light, so far as common 

 objects are concerned. 



99. Spot-lens and Lieberktihn. The spot-lens is used 

 exactly as a substage condenser, and is usually adapted for the 

 T %- objective with amplifier, the amplifier enabling this power 

 to be used by bringing it farther from the slide. Polycystina 

 are beautifully shown by it, but almost too small to be seen at 

 any great distance. Many usual dark-ground objects do not 

 reflect sufficient light, and only experience can decide what 

 are available ; but some slides are best shown in this way. 



The Lieberkiihn is usually fitted for the ^ or 1 inch 

 power, which can be amplified if necessary. It slides upon 

 the objective itself, and must be ordered with it when desired. 

 In using it, all substage condensers are removed, that the 

 parallel beam may fall direct upon the Lieberkiihn, and the 

 lime and the Lieberkiihn must both be adjusted, to get a good 

 effect. Portions of iridescent wings make beautiful objects, 

 also some Foraminifera, but they should not be mounted on 

 an asphalt surface, as, even after the alum-cell, there might 

 be sufficient heat absorbed to soften this. Such asphalt slides 

 are the only ones, owing to their powerful absorption, which 

 it is un advisable to leave in the rays for more than half a 

 minute. When slides can be specially prepared, this is avoided, 

 and the effect improved, by having a disc of black paper on 

 the back of the slide, and mounting the object on the surface 

 of the glass itself. The disc should be as small as will cover 

 the object, in order that as much light as possible may pass 

 round it to the Lieberkiihn. The smaller of those slides 

 prepared with iridescent butterfly scales, in imitation of 

 bouquets, vases of flowers, &c., make very beautiful screen 

 subjects for the Lieberkiihn. 



100. Polarised Light. The addition of polarising ap- 

 paratus largely increases the powers of the instrument. A 

 polarising prism capable of rotation, and large enough to cover 



