eS eg) as fe at lee S E IE A on ea he eR ATEN 
ier a a nt ee EES ee as Se Ne ee i 
Re orf SNe ee ee 
MICROSCOPY. 701 
andth of an inch under a power of one, or seen naturally at ten 
inches. But we can see hairs much finer than this — say three 
times — therefore, with regard to arc, we can see with a power of 
1,000 the 1 of yoyoy, i-e., with a power of 3,000 about the mil- 
lionth. To find the angle in seconds, 1” = 0.000004848 = torvo 
nearly. 
The angle under a power of 3,000, at | 
a distance of ten inches, is for a f me a 
millionth of an inch. 
3. 3 
10 X 1000000 — 10000 
Divide this by the value of one second and we get six seconds 
in the angle subtended by z554$555 under a power of 3,000.” 
Use or MICRO-PHOTOGRAPHS.— The experience of the late siege 
of Paris has given a permanent prominence to microphotography 
as a practically useful art. A French engineer now proposes to 
reduce messages photographically to microscopic size, and then 
blow them through a pneumatic tube under the straits of Dover 
to England, where they should be raised by photography to a legi- 
ble size again. Thus the promptness of the telegraph would be 
approached, while its expensiveness, in the case of long messages, 
would be avoided. 
STRUCTURE or Diatoms.—In stating Mr. Stodder’s disbelief that 
the silicions matter in diatoms was always deposited in spherules, 
we omitted to explain that it was the processes, or so-called feet, of 
E. Argus which he considered structureless. Mr. Stodder also 
desires us to notice that he does not adopt Mr. Slack’s term +‘ ordi- 
nary diatoms ;” that he has not believed the markings on all diatoms 
to be depressions, but that the dark spots seen by reflected light 
on E. Argus are so; and that he and Mr. Wells have not been as- 
sociated in studying this subject. He also contributes the follow- 
ing remarks in regard to test objects and high powers. 
“The histologists, vegetable and animal anatomists may say as 
they have said, ‘What of it? Is it worth the time and labor re- 
quired to determine whether the minute granule of one fifty thous- 
andth of an inch is hexagonal or circular, a pa or a cavity?’ 
Yes, gentlemen, it is, for so long as these questions are unset- 
tled, so long must you be uncertain of the true interpretation of 
your own observations ; so long as you do not use the best instru- 
ments and the highest powers, so long must you be ignorant of 
