ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



367 



a definite size, 2 mm. If this is not done the efficiency of the condenser 

 is reduced — for instance, the small ultramicroscopic particles in the blood 

 cannot be seen by the eye. This drawback led to the modification 

 shown in fig. 58, in which the mirror-lens is replaced by a truncated 

 cone. Inspection of the figure shows the path of the rays. The rays 

 of light are less concentrated on the object, but it is not necessary to 

 keep" to a fixed thickness of the glass slide on which the object is placed. 

 Slips from 1-2*5 mm. thick can be used with equal advantage. This 

 latter condenser is specially recommended where sources of light suffi- 

 ciently powerful are available. The condenser first described gives good 

 results, not only with sunlight and the arc lamp, but also with less 

 intense lights, such as small arc lamps, which can be used on any electric 

 light circuit in place of the ordinary incandescent ones. Nernst lamps 

 also give satisfactory results. Welsbach burners used with compressed 

 coal gas are also practicable for ultramicroscopic work. Fig. 59 shows a 



Fig. 59. 



small arc lamp fitted up with a condensing lens. Fig. 60 shows the new 

 reflecting condenser in a mount which can be used with any ordinary 

 Microscope in place of the Abbe condenser. 



Instructions for Use. — The reflecting condenser has proved of use 

 for examination of (1) colloidal solutions, (2) blood, (3) every kind 

 of unstained living bacteria, (4) transparent solid objects if thin sec- 

 tions can be cut. The most important point is to have the greatest 

 possible cleanliness. Without this good results cannot be obtained. 

 Small particles of dust, scratches, air-bubbles, and other imperfections 

 in the glass slip or cover-glass have a very bad effect. For this reason 

 only very good slides and cover-slips should be used. All objects, 

 liquids, bacteria, etc., are simply placed on the glass slide and covered 

 with a slip. It only remains to have a homogeneous connection, as 

 free as possible from air-bubbles, between the top surface of the 

 condenser and the lower surface of the slide. Cedar oil is best for 



