4S2 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [December 16,1871. 
able association of kindred spirits; lastly, the healthy 
nature of it. I have purposely avoided the beaten 
track of mercantile gain and professional advan¬ 
tages, because I feel that there is in the pleasure to 
be derived more than sufficient to induce a love for 
the study of botany. I never pass down a country 
lane without finding amongst the wayside flowers 
food for enjoyment and contemplation. 
Once more let me commend to all students the 
pe*rusal of Professor Bentley’s two addresses men¬ 
tioned at the commencement of this communica¬ 
tion, written with the view of encouraging the 
young, and reminding them that botany is not 
really the dry subject it has been too frequently de¬ 
signated, or at any rate that it has attached to it 
pleasant memories and associations. 
A CHAPTER IH MICROSCOPY. 
BY HENRY POCKLINGTON. 
(Concluded from page 442.) 
Optical Errors .—Errors of interpretation fre¬ 
quently arise out of certain well-known optical phe¬ 
nomena, and are less easily guarded against, because 
less expected. “ Seeing is believing,” says the old 
saw; but in optics seeing must very frequently be, 
not believing. One of these phenomena we have 
already glanced at in the case of the oil, air and 
water globules. A very similar case is that of the 
lacunae in bone, so long mistaken for opaque ra¬ 
diating solids. Not unlike also are the cases of the 
concavity of the blood-corpuscle, the so-called tubu¬ 
lar structure of the human hair, and the so-called 
hexagonal areolation of the valves of certain Pleuro- 
sicjmce. One rule may serve as guide here. Care¬ 
fully alter the focus in the way we have just described. 
If the portion of the structure under view appear 
brightest when the distance between the objective 
and itself is greatest, and darkest when that distance 
is least, it may, with tolerable safety, be concluded 
that it is a superficial elevation. The rule is, of 
course, modified if the particular portion of the 
structure possess a different refractive power from 
its surroundings; but making allowances for this 
(any variation in this respect may be detected by 
altering the direction of the illuminating pencil), the 
rule may be relied upon with safety. The student 
may practise upon thh eyes of insects, provided they 
be carefully mounted. We would recommend him to 
secure a really good specimen of the eye of a beetle 
(one in our possession, mounted by Mr. J. F. 
Barnett, of Tottenham, is exquisitely done), human 
hair mounted dry and in balsam, and a slide of 
Pleurosigma formosum, and to carefully study these 
under all possible conditions as regards the direction 
of the illuminating pencil. But, after all, the most 
perplexing phenomena are those due to diffraction. 
Perhaps, without plunging into mathematics, we 
may, in a few words, explain the cause of these 
phenomena before we attempt to discover a safe¬ 
guard from their misleading tendencies. 
These phenomena appear to have been first noted 
a little more than two centuries since by Grimaldi, 
of Bologna, but it is only within modern times that 
they have been thoroughly investigated. They are 
due to the fact, that a ray of light is, under certain 
circumstances, indexed in passing the margin of an 
object. The phenomena may be easily observed if 
a diverging pencil of light be permitted to enter a 
dark room through a narrow aperture, and a knife- 
edge be held in the path of the pencil just above 
its point of divergence. The shadow of the ob¬ 
ject will be split down the centre by a bright line 
bordered with fringes of colours in harmonic pro¬ 
gression; or, a grating of fine wires may be ar¬ 
ranged so that no light shall enter the eye but has 
passed through the grating, when exceedingly beau¬ 
tiful effects, dependent upon the size and arrange¬ 
ment of the wires, will be produced. The phe¬ 
nomena are thus explained : * if the diverging 
rays, which are indexed on one side of the pins or 
wires, meet those which are indexed on the opposite 
side in the same phase of undulation (on crest or in 
trough of wave at same time), they coincide and 
produce a line of white light; whilst rays, which 
differ in their path, encounter each other under dif¬ 
ferent phases, and interfere, producing either dark¬ 
ness or a coloured fringe. (The student at the sea¬ 
side, or near a lake or pond, may study these phe¬ 
nomena at his leisure in visible ■waves of water.) 
Very similar in character to diffraction interferences 
are those known as thin-plate interferences, and 
we may conveniently study them together. 
In microscopy we meet with the one when we 
study fine striae, as in diatoms, some of -which give a 
lovely iridescent glow when viewed by an intense 
beam of white light; the others, in such structures 
as the ramenta of Ceterach and certain vegetable 
hairs. Both are met with in certain histological 
preparations, where dne fibres lie side by side, or are 
interlaced, or where two membranes are closely su¬ 
perimposed. Very commonly in the barred, or pitted, 
vessels of plants, it is exceedingly difficult to decide 
■whether the apparent puncture is complete, or whether 
the primal cell wall is still present, but uncovered 
by the later deposit, as it is visible only as a narrow 
band of a faint reddish colour, with, perhaps, a dark 
band round the centre. In a slide of the vessels of 
Tropceolum majus (nasturtium), now before us, there 
is a splendid illustration of this class of phenomena ; 
and it was not without long and careful study that 
the true character of the structure could be made out, 
We did it at last by the use of homogeneous light, 
which, being obviously incapable of interference, 
enabled us to see, by aid of careful focusing, that 
there was really no complete aperture in the cell 
under observation, but that the later deposits of 
sclerogen had left the primal cell wall uncovered in 
those particular portions. This is, of course, a very 
common structure, the less frequent being the per¬ 
forate one. The use of monochromatic light, in all 
cases where there is the least reason to suspect 
interference phenomena, is strongly to be advised. 
Careful use of the polarizer without analyser, but 
with and without selenite, is also, in many cases, of 
great service. The student may familiarize him¬ 
self with these phenomena, and the way of dealing 
with them, by the careful study with low powers in 
a strong light of such things as these,—gratings of 
fine wires, meshes of fine threads or wires, films of 
mica having their surfaces broken up by being bent 
to a right angle and then reflexed, and, lastly, of 
such fine powder as the sporidia of Lycoperdon (puff¬ 
ball), and of various species of Ustilago. He will, 
after a course of such study, be able almost intui¬ 
tively to pronounce as to the true character of the 
* Brookes, ‘Natural Philosophy,’ 6th edition. 
