ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1017 



tube-pieces wliicli slide upon the rod B and are clamped by the screws Oi 

 and O2 ; the lower and shorter of these tubes D carries on the horizontal 

 ribbed arm F, the plate G with the condensers Hi and H., and also the 

 plate J with the Microscope K. To the upper and longer tube E is 

 fixed the light-chamber L with the arc-lamp M screwed to its upper 

 side. 



The light used is that known as the Piette-Krizik lamp which, on account 

 of its accuracy of regulation, has been very largely used, and for this very 

 reason has been better tested than many other systems, and which, in spite 

 of its excellent construction, is moderate in price. Since, however, with 

 the best regulated lamps the point of light after long use is invariably 

 shifted slightly upwards or downwards, the piece which carries the lamp 

 and the light-chamber is made to slide up and down the fixed part which 

 supports the condensers and the Microscope, so as to bring the point 

 of light back into the axis of the condensers. This movement is effected 

 by means of a nut N which works between the parts D and E upon the 

 screw-thread of D, so as to raise or lower the light-chamber L and the 

 lamp M, and bring the point of light X to any desired height. A 

 rotation of E with the light-chamber is rendered impossible by bringing 

 the plate G close against the front side of L and making it fit in grooves 

 upon the front of the light-chamber, so that the two parts can only move 

 upon one another in a vertical dii-ection and keep the source of light 

 completely inclosed. 



The light-chamber is made of strong oak, and to the top is screwed 

 the lamp from which the two iron rods which carry the carbons project 

 into the chamber. In the top are also, besides the aperture for the carbon- 

 holders, several ventilating holes to carry off the hot air; these holes 

 are covered with tin caps to screen the light. The left side of the 

 chamber is entirely closed, while the right side is provided with a door 

 to allow the insertion of new carbons, &c. In the centre of the door is a 

 circular hole closed with dark glass through which to observe the glowing 

 carbon points. At the bottom of the chamber is a circular opening through 

 which the shelf for the objects is illuminated, and which serves for 

 ventilation ; above the opening is a dark glass to moderate the light and to 

 catch the ash which falls from the carbons. 



The front side of the chamber has an aperture into which the condenser- 

 holder projects. This aperture is made large enough to allow free play for 

 the chamber ; the source of light being brought into the axis of the con- 

 densers, as was said above, by a motion of the chamber. In the axis of the 

 lenses Hj and H., is the Microscope K, supported on the plate J which is 

 attached to G. 



For the lenses I use the ordinary horseshoe stand with the following 

 alterations : (1) the upper piece for rotating the Microscope is unscrewed 

 from the foot, turned through 180^, and fixed again to the foot so that the 

 stage is not over the horseshoe, but projects behind it; the horizontal 

 Microscope can then be brought as near as is required to the large con- 

 densing lens Hj, which with low powers is necessary to secure a colourless 

 image. (2) Instead of the small thick stage, a large plate O with 

 diaphragm U and two clips is used ; (3) in place of the tube moved with rack 

 and pinion, there is an arm P, moved in the same way and carrying the 

 nose-piece Q, which allows a rapid change of objectives. A metallic screen 

 E, of 15 cm. diameter, serves to arrest the rays which pass beside the 

 objective ; this is placed immediately behind the nose-piece. 



With high powers the object must be brought near to the focus of the 

 condensers, while with low powers it must be moved beyond the focus and 



