62 MEMORANDA, 
pleurosigmata as a series of hexagonal spaces by which the 
surface of the valves is partitioned. 
“Now, Mr. Woodward tells us that Wenham, abandoning 
his previous judgment, has acknowledged that the conforma- 
tion of the markings is circular. Thus the hexagonal appear- 
ance in Wenham’s photographs would be nothing, according 
to Jabez Hogg, but ‘an exaggerated imperfection produced 
by an error of forx in his lenses.’ 
“These discordant views of the most celebrated micro- 
graphers encouraged me in trying to get a most possibly 
accurate idea on the subject, for the which purpose I examined 
most carefully the photographs of P. angulatum taken by 
myself and by others, too, under different degrees of power, 
giving my especial attention to the negatives on glass, which, 
it is well known, present a greater nicety of details than 
positives. And availing myself of the monochromatic illumi- 
nation, which I always employ in testing the most difficult 
object, I felt convinced that the hexayonal form is the only 
true structural element of the surface of P. angulatum, seeing 
that by a direct observation with light decomposed, the strie 
present themselves always bended (in zigzag) and never 
straight, and that but in three directions—one transverse, 
and two oblique. 
‘© Besides all this, I must confess that I cannot understand 
how an object not perfectly in focus, or excessively magnified, 
may produce the illusion of a circular changed in any other 
angular form, whilst the contrary, it is obvious, succeeds 
whenever the vision of an object is less distinct, say for its 
being too far off, or for interposition of mistiness, or for any 
other cause. 
‘This is my simple manner of viewing the thing. However, 
T should always be glad if anybody would show me that Iam 
mistaken. At any rate I wish they would try the monochro- 
matic light, which I feel confident they will find useful to 
decide this as well as many other difficult points.” 
So far Count Castracane’s letter on the subject. If there 
is in my rough translation any technical or other error, I 
hope you will correct it—Yours very truly, PRoFEssor 
JosEPH GazLIARDI, Cardiff. 
Microscope Lamp.— Mr. John Bockett sends us a photo- 
graph of his method of mounting and using a microscope 
lamp. A pillar upon a foot carries a glass lamp with a 
reflector behind it, and a condensing lens in front. The 
