T. F. SMITH ON THE VALVE OF PLEUROSIGMA. 307 



Fig. 3. Upper plane of red " beads," thrown by each alternate hole of 

 grating, Fig. 2 ; on lowering the focus white interspaces 

 turn into blue beads. 

 „ 4. Lower plane of bine beads thrown from remaining holes of 

 grating shown in Fig. 2; on raising the focus white inter- 

 spaces tnrn into red beads. 

 „ 5. Middle layer of diagonal grating showing torn structure. 

 „ 6. Fragment of middle layer found on same slide. 

 „ 7. Middle layer in sound valve lying immediately under delicate 

 membrane, similar to that shown in Fig. 1. 

 Object glass used, Zeiss's apochromatic oil immersion, T ^ inch N.A.1.40, 

 magnifying power 1,750 diameters. 



Journ. Q. M. C, Series II., No. 23. 



23 



