ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



325 



the fixed diaphragm of tho ocular up or down, and a millimetre scale 

 and graduations on the flange of the diaphragm itself permit an adjust- 

 ment to tenths of a millimetre. The focus, once found, should be note J, 

 and can then always be recovered. In another arrangement he pushes 

 the fixed diaphragm down on to the mount of the lower lens, and the 

 movable diaphragm has been so modified (fig. 49) that it rests directly 

 on the displaced diaphragm. The movable diaphragm is slightly altered 

 in construction so as to carry a millimetre scale, and its arrangement 

 will be easily understood from the figure. 



In the second type (fig. 50) the three vertical springs are reduced to 

 one, which is rather broad, and its curved extremity is useful for hand- 

 ling the diaphragm. Moreover, this vertical stem is provided with a 

 screw thread, by means of which it may be screwed up or down inside 

 a short tube, which acts as its holder and is dropped down on to the 

 mount of the lower lens. Vertical springs set in the thickness of this 

 tube serve to keep it steady in its place. There is a vertical millimetre 

 scale, and the flange of the diaphragm is also divided, so that adjustment 

 is possible to the tenth of a millimetre. A vertical window permits the 

 exact position of the glass slip to be known. 



(3) Illuminating and other Apparatus. 



Leitz' Large Projection Apparatus. — In this apparatus (fig. 51) 

 the Schuckert electric projection lamp is employed. The lamp requires 



Fig. 51 



a constant electric current of 12-20 amperes, and connected with it is a 

 triple condenser of 150 mm. diameter. Various contrivances permit 

 very accurate adjustment of the lamp and condenser. For example, two 

 thumbscrews control the positions of the carbons, so that the luminous 

 point may be readily brought into the exact optical axis of the con- 

 jee 19th, 1901 z 



