281 Markings of Frustulia Saxenica. By Dr. J. J. Woodward. 
mounted the specimen in Canada balsam. The striae were then 
paler than before, but I cannot say that I found them more 
difficult to resolve. Both in balsam and dry I could get resolu- 
tion by the Tolles’s immersion |th belonging to the Museum, and 
that by lamplight as well as by monochromatic sunlight. With 
immersion objectives of higher powers the lines were still more 
distinctly separated, and I obtained the finest results with the 
immersion front of the of Powell and Lealand, and with 
the new immersion Wth recently made for the Museum by 
Mr. Tolies. 
On the whole, the Frustulia Saxonica is an easier test than 
the Amphipleura pellucida, as may he inferred from the above 
measurement of its strife, and the difference is especially marked by 
lamplight. Those therefore who work by lamplight only will find 
this test more extensively useful than the Amphipleura. 
The Woodbury print [see Plate CXXIV., Fig. A] illustrating 
this article is copied from a negative made by the immersion front 
of the Powell and Lealand Wth belonging to the Museum. The 
power used was 1750 diameters. It displays the transverse strife 
as seen when the utmost pains are taken to avoid the longitudinal 
diffraction lines. 
London : Printed by William Clowes k Sons, Stamford Street and Charing Cross. 
