372 



Introduction to Botany. 



depth of I inch — a hole of slightly smaller diameter having first been 

 bored to receive it. Spring brass wire of /j-inch diameter is then cut 

 into lengths of 17 inches, and each piece shaped to serve as two lens 

 holders according to the following directions : — ■ 



Clamp one end of a wire nail of about JJ-inch diameter firmly in an 

 iron vise. Place the brass wire with its middle against the wire nail, 

 and wrapping from both ends, make a coil of ten turns about the nail, 

 and cut the wire in two at the middle of the coil ; then at i J inches from 

 the center of the coil of each half bend the wire into a loop to receive 

 and firmly hold the doublet lens. When completed {d), the spiral coil 

 should grip the rod in the block firmly enough to hold the lens at any 

 height, but without binding too tightly to prevent an easy vertical 

 adjustment. The block should be given one or two coats of shellac 

 into which enough lampblack has been stirred to make a black varnish. 

 No glass stage or mirror is necessary, for the shellac affords a black 

 background against which thin sections, etc., are strongly contrasted. 



238. Dissecting Needles. — Dissecting needles can be made as fol- 

 lows : Grasp a strong needle in a pair of pliers and thrust it, eye 



foremost, into a handle of soft wood 

 or into a smooth twig Avith very small 

 pith. Hard wood handles, such as 

 can readily be prepared from butch- 

 ers' skewers, should have holes drilled 

 into them to receive the needles. A 

 good drill for this purpose is made by 

 breaking the eye from a needle a size 

 larger than those which are to be in- 

 serted, and rubbing the broken end 

 into the form of a drill point on an 

 oil stone. The needle should be 

 pushed into the handle a slight dis- 

 tance beyond the bottom of the hole 

 made with this drill. 



239. Manner of Using Lenses. — 



For much of the work requiring the 



use of a lens the dissecting block will 



not be needed, the lens simply being 



held between the thumb and forefinger of one hand while the object 



is held in the other hand in the same manner. To prevent vibration, 



Fig. 197. 



Showing the manner of holding lens 

 and object. 



