DISSECTING SPECIMENS. 199 



Dissecting Microscope will be found a handy and use- 

 ful form, fig. 138. 



This instrument consists of a duplex lens of three 

 powers, magnifying 4, 6, and 10 diameters, screwed to 

 the end of a brass focussing tube, and moving upon a 

 brass pillar attached to a sliding bar at the bottom 

 of the box. The dissecting stage is a cork slide, plain 

 on one side for general work, and a shallow cell on the 

 other, for the dissection of such objects as small glossy 

 seeds which "fly" under the needles, whilst a pitted 

 glass slide, for carrying on dissections under water, 

 is also provided. 



Microscopic Dissection. The mode of using needles 

 for teasing out tissue is very simple. With a pair 



FIG. 138. Browning's Houston Botanical Dissecting Microscope. 



of the small needles held firmly between the fore- 

 finger and thumb, as shown in fig. 139, the structure 

 must be teased out ; an operation which requires some 

 care. All substances should be carefully separated 

 from dust and other impurities which render their 

 structure indistinct or confusing. With delicate mem- 

 branes, those of the nervous system of the smaller 

 animals, insects, etc., it is necessary to make the dissec- 

 tion under water, or in fluid of some kind. For this 

 purpose take a glass cell, and then throw a strong light 

 down upon it by the aid of condensing lens, as repre- 

 sented in fig. 140. Delicate structures will be better 

 teased out in a dilute solution of sugar or common salt, 

 to prevent change from endosmose. Should the object 



