170 Transactions of the Boyal Microscojpical Society. 
in very oblique light produce diffraction phenomena, which in some 
cases obscure the whole frustule, and usually leave but little of the 
real aspect perceptible; balsam, however, has a refractive power 
more nearly that of the silica, and in that medium the diffraction 
lines are quite within control. 
The illumination which I have used in taking this photograph 
was obtained from Wenham's reflex illuminator, used in the manner 
first described by me in the * Boston Journal of Chemistry,' and 
reprinted ' M. M. J.' vol. xiv. p. 30. Of course only very wide- 
angled objectives can be used with the reflex in this manner. The 
one I have chosen for the purpose is a ToUes' y^th duplex front, 
made for me last year. 
I send also, marked B, a photograph of the transverse lines of 
the same frustule, and one, marked C, of the Amjphipleura pellucida, 
No. 20, on the same Probe-Platte, both taken with the same ob- 
jective and illumination. They appear to indicate the beaded 
character of the striation. 
I have seen no record of any photograph of the Amphipleura 
pellucida in balsam, except that of Count Castracane,* which he 
did not deem good enough to print from. Following the example 
of that careful observer, I use a prism, and take my photographs 
with the blue rays, 
I send also other photographs taken with central light, which 
1 shall be pleased to have presented to the Koyal Microscopical 
Society, if you deem them of sufficient interest. 
* ' M. M. J.' vol. v., p. 176. 
