Proc. Roy. Soc. Edin.] 
[Vol. XXXIII. 
Fig. 1 . — Coarse ground glass, x 800. 
Fig. 2. — Ordinary ground glass, x 800. 
Fig. 3. — Extra-fine ground glass, x 800. 
Fig. 4. — Acid-etched ground glass, x 800. 
Dr James Robert Milne. 
[Plate I. 
