SCIENTIFIC SUMMARY. 
433 
MICROSCOPY. 
Mr. Sorby on Count Castracane' s views on ultimate Vision with the 
Microscope. — The President of the Microscopical Society (Mr. W. C. Sorby) 
gave a valuable address this year, in which he referred to and elucidated 
Helmholtz’s views. This address was subsequently read by Count Castracane, 
who made a series of observations, showing that vision extended further than 
Helmholtz’s views would allow. These are now commented on as follows 
by Mr. Sorby: — “With reference to the facts here described by Count 
Castracane, I wish to offer a few remarks. It appears to me that the 
visibility of the fine lines of Nobert’s test-plates depends on several different 
circumstances. The light must be thrown in such a manner as to be defi- 
nitely intercepted by the marking on the glass, or they could not possibly 
be seen ; and we have then to consider the effect of interference fringes, as 
well as the quality of the microscope itself. 1 do not see that there 
ought to be any serious difficulty in explaining, on Helmholtz’s principles, 
the resolution of Nobert’s nineteenth band. With such an illumination as that 
adopted by Count Castracane, it appears to me very probable that the inter- 
ference fringes would so far coincide with the true lines as not to prevent 
a satisfactory definition. At the same time I am anxious to make it fully 
understood that in my address I endeavoured more to point out the results 
that would follow from Helmholtz’s theory, than to examine whether it is 
oris not in every respect true. I should be one of the last to wish it to be 
looked upon as a final solution of the problem. I think many questions 
remain to be cleared up by the actual observations of persons conversant 
with the theory, and accustomed to the practical use of high powers. I am 
also inclined to believe that several crucial tests ought to be examined. 
Amongst these I would especially suggest the study of fine lines at very 
close yet unequal intervals, and of lines at equal intervals with one or two 
missed out here and there. Theory indicates that such tests would be far 
more difficult to see correctly than lines ruled at regular and equal intervals : 
and an examination of such tests ought to afford much information respect- 
ing both the final powers of our microscopes and the physical constitution of 
light itself. Helmholtz, of course, assumes the truth of the undulatory 
theory ; and though in the highest degree probable, it would perhaps be 
premature to conclude that it is absolutely certain, when applied to the 
explanation of every phenomenon, especially in such a case as that under 
consideration.” 
Diatomacece Absorbed in their Entire State by the Roots of Plants . — 
Some very curious observations have been made by Professor P. B. Wilson, 
of Baltimore, U.S.A., which seem to show that the Diatomaceee, when 
applied to the earth in which corn was grown, absolutely passed in their 
entire condition through the roots, and were found in the stems of the corn. 
In “ Silliman’s American Journal ” (quoted by the u Monthly Microscopical 
Journal,” August 1876) Professor Wilson says: — “To demonstrate this 
theory, my friend G. I. Popplein, Esq., of this city, suggested the applica- 
tion of infusorial earth of the Richmond formation — found in large quantities 
on the western shore of the Chesapeake Bay — to land sown in wheat. I 
F F 
VOL. XV.— NO. LXI. 
