ZOOLOGY AXD BOTANY, inCKOSCOPY. ETC. 



675 



the cylinder. The part to be examined is placed between the cylinder 

 and the glass plate, which is lowered as far as is possible without 

 causing compression of the tissue between it and the edge of the 

 cylinder ; G represents the web seen in section, M is the stage of the 

 Microscope, and N the objective. 



Fig. 156. 



The membrane D must be sufficiently transparent to allow of the 

 capillary circulation being seen clearly, even with high-power 

 objectives. It must be flexible enough to transmit equally to the 

 tissue examined the pressure of the air within the cylinder, and it 

 must be as inelastic as possible, or, at all events, its elasticity coefficient 

 (to use a convenient but scarcely accurate term) must be considerably 

 greater than that of the tissue against which it presses, so that the 

 latter may not be stretched to any important extent when the pressure 

 in the cylinder is raised. The manner in which it is fastened on the 

 cylinder must also be such that the pressure acting on that part of the 

 web, &c., which lies within the field of the Microscope, i. e. the part 

 lying in the centre of the area of contact between the web and mem- 

 brane, will, when the instrument is arranged as in Fig. 156, be exactly 

 equal to the pressure of the air within the chamber or cylinder A. 



The first three of these conditions are fulfilled to perfection by the 

 membrane which the author used fi'om the peritoneum of the calf, and 

 used by di-uggists for fastening the stoppers of perfume bottles (not 

 to be confounded with the much thicker membrane also used for the 

 same purpose). 



Fig. 157. 



The manner in which the membrane must be fastened on the 

 cylinder A, is illustrated by Fig. 157. A piece of the membrane having 



