OPTICAL PRINCIPLES.
We shall now devote a few pages to the consideration of the nature of
light, and the optical principles involved in the construction and use
of the microscope. Two theories of light have been propounded. According
to one, light consists of minute particles emanating from self-luminous
bodies, as the sun, a candle, or a red-hot piece of iron; this is called
the corpuscular theory. According to the other, light consists of waves
or undulations like those of water or the ears of corn set in motion by
the wind, of the molecules of an extremely subtle and rarified elastic
matter, called ether, existing everywhere, and set in motion by the
causes which produce light; this is called the undulatory theory. The
consideration of the merits of these two theories would be foreign to
our purpose: suffice it to say that the evidence in favour of the
undulatory theory preponderates, so that the corpuscular theory is now
laid aside.
It will often be requisite to make use of the term ray of light, by
which must be understood the smallest bundle of luminous undulations
which can be separated from a mass of light--as by passing light through
a small hole in an opake body, or by any equivalent method.
The most casual observer must have noticed that the rays of light move
in straight lines; as when the sun’s rays are seen entering a dark room
through a small window or other aperture, their direction being then
distinctly visible; the manner in which ordinary shadows are formed also
illustrates the same fact.
_Refraction._--But when the rays in their passage impinge or are
incident upon and enter a transparent medium or material, of a different
density from that which they were at first traversing, their course
becomes altered, and the line of their direction broken, whence they are
said to be refracted. If the medium upon which the rays impinge be
denser than that through which they were at first passing, they will be
refracted towards a line perpendicular to the surface, or they will be
refracted towards the perpendicular, as it is expressed.
Thus, as shown in Pl. XII. fig. 1, the incident ray _i_, entering the
plate of glass, will be refracted at its surface in the direction _a r_,
towards the line _p_, which is perpendicular to the surface.
The extent to which the rays undergo refraction depends upon the degree
of density of the medium, and varies in the case of each individual
substance; but it follows a definite law. If, as in Pl. XII. fig. 2, a
circle be drawn around the point _b_, at which the ray _a_ is incident,
_b r_ representing the refracted ray, the lines _s i_ and _t r_, drawn
at right angles to the perpendicular _p_, will form respectively the
sines, as they are called, of the angles _s b i_ and _t b r_; _s i_
being the sine of the angle of incidence _s b i_, or the angle formed by
the incident ray with the perpendicular, and _t r_ the sine of the angle
of refraction _t b r_, or of that formed by the refracted ray with the
perpendicular. These sines, for brevity, are called the sines of
incidence and of refraction; and they bear a constant ratio or
proportion to each other. Taking the sine of refraction as the unit, or
as = 1, the value of the sine of incidence represents the refractive
index or the refractive power of the medium for a ray entering the
medium from a vacuum; or, the refractive power of air being extremely
small, the value of the sine of incidence may be considered as
representing the refractive power from air into the medium.
PLATE XII. [PAGE 168.]
OPTICAL PRINCIPLES.
Fig.
1. Refraction through a glass plate.
2. Law of refraction.
3. Reflexion from a plane surface.
4. Reflexion from a concave mirror.
5. Refraction at a curved surface.
6. A doubly convex lens.
7. A plano-convex lens.
8. A doubly concave lens.
9. A plano-concave lens.
10. A concavo-convex lens, or meniscus.
11. Refraction through a convex lens.
12. Relation of a convex lens to prisms.
13. Relation of a concave lens to prisms.
14. Refraction of parallel rays through a convex lens.
15. Refraction of converging rays through a convex lens.
16. Refraction of diverging rays through a convex lens.
17. Refraction of parallel rays through a concave lens.
18. Spherical aberration.
19. Dispersion and formation of a spectrum.
20. Chromatic aberration.
21. Formation of images in the eye.
22. Angle of vision.
23. Objects too near the eye.
24. Action of convex lens in vision.
25. Aplanatism.
26. Aberration produced by cover.
27. Course of rays through the microscope.
28. Achromatism.
29. Waves of light conspiring (_a_, _b_), and interfering (_b_, _c_).
30. Polarization: _t_, tourmaline; _d_, crystal; _s_, crystal of
calcareous spar.
[Illustration: Plate XII.
W Bagg sculp
London: John Van Voorst.]
Although the ratio of the sines is constant, the refractive index varies
in different media. Thus that of air is 1·0003; of water, 1·336; of
Canada balsam, 1·549; of crown glass, from which window-panes are made,
1·535; of flint glass, from which bottles are made, 1·6; of Faraday’s
heavy glass, composed of silicated borate of lead, 1·8; and of that
consisting of borate of lead, 2·0.
A knowledge of this “law of the sines” is of practical importance in
determining the direction which the rays will pursue when transmitted
through glass lenses, &c. the refractive index of which is known; or in
ascertaining the curve which should be given to their surfaces for
producing a particular refraction and focal length. Thus, supposing the
plate of glass in Pl. I. fig. 2 to consist of crown glass, the
refractive index of which is 1·5, the length of the sine of refraction,
_t r_, will be equal to one part or dimension, while the sine of
incidence, _s i_, is equal to one part and a half.
It must be remarked that when light is incident at a right angle to the
surface of the medium, no refraction takes place, the transmitted ray
pursuing its original course.
When a ray of light leaves a denser medium, such as glass, to enter a
rarer medium, such as air, it becomes refracted from the perpendicular.
In such case, the angle of refraction being greater than the angle of
incidence, its sine will also be greater than that of the latter; but
the ratio is still preserved.
_Reflexion._--When rays of light fall upon a plane surface, as the flat
surface of the mirror, a greater or less number of them are reflected,
and this according to a definite law, by which the angle of incidence,
or that formed by the incident ray with the perpendicular, is equal to
the angle of reflexion, or that formed by the reflected ray with the
same. Thus, as shown in Pl. XII. fig. 3, the angle _i b p_, formed by
the incident ray _i b_ with the perpendicular _p_, is equal to the angle
_p b r_, formed by the reflected ray _b r_ with the perpendicular _p b_.
If the body upon the surface of which the rays are incident be
transparent, some of the rays will be refracted and will pass through
it, whilst others will be reflected. The proportion of those reflected
is smallest when the rays are incident perpendicularly to the surface;
but this increases as the incident rays become more oblique, _i. e._ as
the angle of incidence becomes greater, although at no degree of
obliquity are the whole of the rays reflected. The case is different,
however, with those rays which enter the substance and impinge upon its
inner or second surface; for these at a particular angle of incidence
undergo total reflexion, so that none of the rays are transmitted at the
second surface. The angle of total reflexion is constant for the same
medium, but different for different media: thus in crown glass it is
equal to about 40°, in flint-glass 38°, &c.; and this internal reflexion
from the second surface of transparent media is more perfect than that
occurring at the surface of opake reflecting surfaces or mirrors.
If the reflecting surface be concave, as in Pl. XII. fig. 4, parallel
rays will be reflected to a focus _a_, nearer the mirror than the centre
of its curvature _b_, and this focus is called the principal focus;
while diverging rays are brought to a focus nearer the centre of
curvature; and converging rays form a focus further from the centre of
curvature.
_Lenses._--In most instances, as far as the microscope is concerned, the
surfaces of the glass through which the rays of light are transmitted
are not plane or flat, but curved-being either convex or concave, and
belonging to convex or concave lenses. In considering the course of rays
through curved surfaces, the refraction may be viewed as taking place at
a plane surface forming a tangent at the point of incidence of each
ray; or each curved surface may be regarded as consisting of a number of
minute plane surfaces placed at right angles to the perpendicular. Thus,
in Pl. XII. fig. 5, the ray _a_, incident at the point _b_ of the curved
surface, is refracted towards the perpendicular _p_, as if it had fallen
upon the plane surface represented by the tangent _t_. The forms of the
most common lenses are represented in Pl. XII. figs. 6-10;--fig. 6 being
doubly convex, or both surfaces being convex; fig. 7, plano-convex, or
one surface plane, the other convex; fig. 8, doubly concave, or both
surfaces being concave; fig. 9, plano-concave, one surface being plane,
the other concave; and fig. 10 is a meniscus, in which one surface is
convex and the other concave. The curved surfaces of lenses are usually
portions of spheres.
The manner in which the course of a ray may be traced through a lens is
illustrated by Pl. I. fig. 11, which requires no explanation after what
has been already stated.
To facilitate the comprehension of the general action of lenses, they
may be regarded as composed of two triangular prisms, with their bases
in contact in a convex lens, as in fig. 12; their apices being opposed
in a concave lens, as in fig. 13.
The point to which the rays converge after passing through a convex lens
is called the _focus_ (Pl. XII. fig. 14 _f_), the distance of which from
the centre of the lens, called the focal length, obviously depends upon
the direction of the incident rays. When these are parallel, which those
coming from distant objects may be considered to be, the focus at which
they meet is called the principal focus, or the focus for parallel rays:
thus, in Pl. XII. fig. 14, the parallel rays meet at _f_, which is the
principal focus.
If the incident rays are convergent, as in Pl. XII. fig. 15, the focus
_o_ will be situated nearer the lens than the principal focus, _f_. If,
on the other hand, they are divergent, as in Pl. XII. fig. 16, the
focus _f_ will be situated further from the lens than the principal
focus _o_. By concave lenses the incident rays are rendered divergent,
as in Pl. XII. fig. 17, as if they emanated from a point _f_, situated
on the same side of the lens as that upon which the rays are incident,
and called the virtual focus.
_Spherical aberration._--Although, as a general expression, we have
stated that the rays of light meet at a focus on passing through a
convex lens, this is not strictly correct. For, in ordinary convex
lenses, the marginal rays are more refracted than the central ones, and
meet at focal points nearer the lens than the latter, as shown in Pl.
XII. fig. 18. This important defect is called spherical aberration, and
arises from the lateral rays being incident upon more oblique portions
of the curved surface of the lens than the central rays. Hence objects
seen through such lenses appear misty and confused, the central and
lateral parts of a flat object not being visible at the same time; and
even when the marginal parts are visible, they appear distorted or
deformed.
Spherical aberration is greatest in the most convex lenses; and, in a
plano-convex lens, it is least when parallel rays enter at or emerge
from its convex surface.
In certain lenses, the convex surface of which has the form of a
parabola, a hyperbola, or an ellipse, the spherical aberration is
absent; but it is impossible to grind microscopic lenses of these forms
with absolute accuracy, so that the fact is of no practical value.
The form of simple convex lens most free from aberration is that in
which the curves of the two surfaces form parts of a sphere, the radii
of the curves being as 1 to 6; the focal length being rather less than
twice the length of the radius of the most convex surface. This form of
lens comes very near to a plano-convex lens, which is consequently the
best form for a simple lens.
_Dispersion, or Chromatic Aberration._--The rays of light have so far
been considered as simple. They are, however, in reality compound,
consisting of a number of primary-coloured rays, of which seven kinds
are easily distinguishable, viz. red, orange, yellow, green, blue,
indigo, and violet. The coloured rays of the sun are, as is well known,
often seen separated by the action of the triangular glass bars or
prisms forming the lustres of a chandelier; the separation arising from
the different refrangibility of the coloured rays, by which each is
refracted to a different degree from that of the others. This is shown
in Pl. XII. fig. 19, representing a ray of white light entering a
triangular prism, at the surface of which the paths of the rays become
different according to the degree of their refrangibility, whence they
emerge separately, forming a _spectrum_ at _v r_; the most refrangible
violet rays (_v_) being most refracted, the less refrangible red (_r_)
least so, the intermediate rays being refracted to intermediate degrees
according to their respective refrangibilities. This separation of the
coloured rays is called dispersion; and as different substances or media
disperse the coloured rays over a larger or smaller space, so as to
produce spectra of different lengths, they are said to possess different
dispersive powers. Thus the dispersive power of flint glass and balsam
are about equal, while that of crown glass is considerably less.
The extent to which dispersion is produced by the same medium also
depends upon the angle of the prism, being greater as the angle is
larger; increased obliquity of the incident light also increases the
dispersion, so that the spectrum produced by a small prism may be equal
to that produced by a larger one upon which the light is less obliquely
incident.
In consequence of the dispersion of light, rays passing through a
convex lens do not meet at a single point or focus (Pl. XII. fig. 20),
but form as many foci as there are coloured rays.
When the spectrum is received upon a convex lens, the coloured rays are
brought to a focus, and the light appears again white; for it is only
when the primary-coloured rays are parallel, and seen close together,
that they produce the impression of white or colourless light. The
spectra produced by different dispersive media not only differ in
length, but also in the breadth of the coloured spaces not being in the
same ratio to each other; hence the spectra are said to be irrational,
or the dispersion is said to be irrational.
_Vision._--The visibility of an object depends upon the rays of light
which emanate from each point of its surface presented to the eye being
brought to a focus upon the ret´ina or expansion of the nerve of sight
lining the inside of the back of the eye; so that, an image of each
point being impressed upon the retina, the sum of the images forms the
compound image of the entire object.
The manner in which the image is formed is shown in Pl. XII. fig. 21, in
which, to prevent confusion, the rays coming from three points of the
arrow only have been represented. The rays diverging from these three
points, _a b c_, form cones in contact by their bases; the apex of each
cone outside the eye being situated at the points _a b c_, the common
base of each being situated at the crystalline lens _x_, immediately
behind the pupil or rounded aperture in the coloured curtain of the eye,
called the iris, _i i_, and which limits the base of the cones. The
apices of the cones within the eye, _a´b´c´_, are formed by the rays
brought to foci upon the retina by the crystalline lens. The marginal
cones of rays coming from the object cross within the eye, so that the
uppermost rays from the object become lowermost upon the retina, and
thus an inverted image of the object is formed. This appears to the eye
to be erect, because the eye or rather the mind judges the parts of an
object to be situated in that direction in which the rays coming from
them are impressed upon it. Hence, as in fig. 21, the rays impressed
upon the retina at the lower part _a´_, appear to come from the upper
part of the cross, although they are lowermost in the image, and so on
for the other rays. For distinct vision the rays of each cone must be
parallel, or nearly so.
_Angle of Vision._--The marginal rays coming from the object cross at a
point corresponding to the centre of the pupil, and thus form an angle,
as seen in fig. 22, where the cones are omitted, to avoid confusion;
this angle is the angle of vision. Now the size which objects appear to
possess is measured by this angle, or by the linear magnitude of their
images (_i. e._ their size estimated in one direction, as of length or
breadth) upon the retina. When the object is distant, the angle and its
linear magnitude are small, and it appears small and distant; whilst if
it be large, or if small and brought near the eye, the reverse will be
the case.
_Magnification._--Hence an object may be made to appear larger, or may
be magnified, by increasing the linear magnitude of its image upon the
retina, which can be done by bringing it nearer the eye, as shown in
fig. 22, where the image of _b_ formed at _b´_ is larger than the image
of _a_ formed at _a´_. But when an object is brought nearer the eye than
about 8 or 10 inches (for the distance varies with different persons),
its image becomes indistinct and misty; and this because the rays
composing the cones are too divergent to meet at a focus upon the
retina, as shown in fig. 23. By interposing a convex lens, however,
between the eye and the object, the too divergent rays may be made to
meet at a focus upon the retina, as in fig. 24, the object at the same
time being rendered apparently larger or magnified, from the refracting
action of the lens upon the cones.
_Aplan´atism_ (_a_, not, πλανἁω, to wander).--The effect of spherical
aberration in rendering the image of an object seen through a lens
indistinct and misty may now be intelligible. In order that such image
may be distinct, the rays emanating from each point of the object must
converge at one spot upon the retina. But since, when spherical
aberration exists, the marginal rays are more refracted than the central
ones, they will meet at foci before those formed by the latter; and when
the foci of one set are coincident with the retina, so that the image
would otherwise be distinct, the latter is rendered confused and
indistinct by the rays of the other set.
In this consideration, we imply that there are only two sets of rays,
the central and the marginal; but the central and marginal rays are not
separate, for the rays possess every intermediate degree of obliquity,
hence the foci and images are really innumerable.
Now there are evidently two methods of destroying or correcting
spherical aberration, viz. by excluding the marginal rays, or by
altering their direction.
The exclusion of the marginal rays is often adopted; and is effected by
means of a diaphragm, or stop as it is called. This consists of a plate
of metal, with a round aperture in the middle, and it is placed behind
the lens; but it has the serious defect of diminishing considerably the
amount of light transmitted.
The alteration of the direction of the marginal rays is produced by
refraction, a thin plano-concave lens being placed in front of the
convex one (Pl. XII. fig. 25). The doubly convex lens is composed of
crown glass, and the concave lens of flint glass, which has a higher
refractive and dispersive power than crown glass. In this way we get a
compound lens, which, if the two lenses had the same refractive power,
would simply amount to a plano-concave lens with the marginal portions
removed. But as the concave lens consists of more highly refracting
material than the convex, if the curve and thickness of the two lenses
be properly adapted, the marginal portions of the concave correct the
too great convergence of the marginal rays produced by the convex lens,
and so the rays are brought to nearly the same focus. An idea of this
action may be obtained from fig. 25, the dotted lines indicating the
direction which the rays would take, if passing through the convex lens
only.
A lens in which the spherical aberration is corrected is said to be
aplanatic.
_Achrómatism._--Supposing the spherical aberration of a lens to be
corrected, there still remains the chromatic aberration (p. 173); for
although the central or mean coloured rays may meet at a focus, the
other coloured rays belonging to the same compound or ordinary ray will
meet at different foci, so that a series of coloured images of the
object will be formed at different distances from the lens; hence, at
whichever focus the object is viewed, it will appear coloured.
Now the coloured primary rays can only be made to coincide in direction,
so that the light parts of an object may appear white, by refraction.
And the correction is produced by the same plano-concave lens as that
which corrects the spherical aberration. But in this case the relative
dispersive powers of the media composing the convex and the concave
lenses form the point to be considered. If the dispersive power of the
media of which the convex and concave lenses are composed were the same,
the dispersive power of the convex lens would be in excess, and the
coloured rays in each compound ray could not become parallel. But by
forming the concave lens of a more highly dispersive medium, with a less
proportional mean refraction than the convex, when the curves of the
surfaces and the relative thickness of the lenses are properly adjusted,
the dispersive action of the concave lens may be made equal to that of
the convex; and being exerted in the opposite direction, the coloured
rays will become parallel and meet at a single focus.
This may be elucidated by considering the lenses as composed of prisms.
Thus, let fig. 28 represent the compound lens, the two halves of the
doubly convex lens acting as two triangular prisms (fig. 19) with their
bases opposed, converging the compound white rays _w w_, and dispersing
the coloured elementary rays, which would form spectra at _s s_. In the
plano-concave lens the triangular prisms may be considered as placed
with their apices towards each other, and so would tend to disperse the
coloured rays in the opposite direction, to form spectra at _t t_. Then,
supposing the dispersions to be equal and in opposite directions, the
coloured rays would become parallel and meet at a definite focus, the
colour being destroyed. At the same time, the spherical action of the
concave lens being opposite to that of the convex, the converging action
of the latter will be diminished, so that the focus of the compound lens
will be longer than that of the convex alone; but as the dispersive
power of the concave is greater relatively than that of the convex, the
mean refraction is less altered than the refraction or dispersion of the
separate coloured rays; so that the concave wholly opposes or corrects
the dispersion produced by the convex, while it only partially corrects
its mean refraction.
A lens in which the chromatic and spherical aberrations are corrected or
destroyed is commonly called achromatic; although the term properly
applies to the correction of the colour only.
If in a compound lens the chromatic aberration is only partially
corrected, so that the red rays still meet at a focus beyond the violet,
as in a simple uncorrected lens (fig. 20), the lens is said to be
under-corrected, or the aberration to be positive; while if the
correcting action of the plano-concave lens be too great, so that the
violet rays meet before the red, as in a simple concave lens, the lens
is said to be over-corrected, and the aberration is called negative.
Although the positive chromatic aberration of the extreme rays passing
through a convex lens may be corrected by the negative aberration of a
concave lens, there still remains a certain amount of uncorrected
colour, arising from the irrationality of the spectra of the two
refracting media. This evil cannot be overcome, and the remaining colour
is said to arise from the secondary spectrum.
The object-glasses of the microscope, consisting of the compound lenses,
have their aberrations balanced to a considerable extent on the above
principles--the lowest combination being under-corrected, while the
upper combinations are over-corrected; and, by suitable adaptation of
their distance from each other, further correction may be obtained, the
aberration of the object-glass altogether being, however, over-corrected
or negative.
The eye-piece consists of two simple plano-convex lenses, the upper or
eye-glass (fig. 27 _e_) having a shorter focus than the lower (_f_) or
field-glass, and the two placed at the distance of half the sum of their
focal lengths. The object-glass alone would form an enlarged and
reversed image of the object within the body of the microscope, the
cones of rays from each point of the object terminating at the larger
arrows in the figure (fig. 27). But the rays meeting the field-glass are
brought by it to a focus at the position of the smaller arrows, where
they form a reduced image; and, subsequently passing through the
eye-glass, they are so altered in direction as to enter the eye at a
greater angle, and to present a magnified image of the object.
The eye-piece produces several important effects. The refraction being
produced by two less convex lenses instead of one of greater convexity,
the spherical aberration is considerably reduced; and the convexities
of the lenses in the eye-piece being situated in an opposite direction
to that of those in the object-glass, the spherical aberration of the
former reverses and so neutralizes that of the latter. Also the
under-correction of the field-glass compensates the over-correction of
the object-glass--the blue rays which are refracted more than the red by
the field-glass, being thrown upon the eye-glass nearer its centre,
where the refraction is less, and thus the coloured rays become parallel
or nearly so on reaching the eye. Moreover the field-glass collects a
larger number of rays than the eye-glass could do alone, so that it
enlarges the field and increases its brightness.
In the best object-glasses the aberrations are so well balanced that the
mere covering an object with thin glass is sufficient to disturb the
balance and render very delicate markings either misty and coloured or
wholly invisible. The effect produced by a plate of glass may be
understood by reference to fig. 26, the rays being supposed to emanate
from the object at _a_; and it is evident that the refraction of the
glass so alters the direction of the rays that they will fall upon the
lower combination nearer the centre than if the cover were absent, and
thus negative aberration is produced. In the best object-glasses,
however, this aberration may almost entirely be removed, the lower
combination being susceptible of approximation by a screw movement to
the second or next above it, so that the ascending rays, being able to
continue their oblique course through the increased distance between the
object and the lower combination, may fall upon the same portions of the
latter that they did before the cover was applied.
POLARIZATION OF LIGHT.--In attempting to give a sketch of this curious
and difficult subject, we must suppose the reader to be in possession of
a natural crystal of calcareous spar, and either two Nicol’s prisms
(forming the ordinary polariscope) or two plates of the mineral called
tourmaline cut in the direction of the length or axis of the crystal.
Hitherto we have considered rays of light falling upon transparent
substances as simply refracted or reflected according to the ordinary
laws of refraction or reflexion. We have now to notice some curious
exceptions, forming the basis of many interesting phenomena, especially
in connexion with the microscope, in which these laws are more or less
deviated from. If we place a plate of tourmaline, cut as above directed,
upon or beneath the stage of the microscope, the light will pass through
it, appearing tinged with the green or brown colour natural to the
tourmaline; but on laying another slice upon the eye-piece, and turning
the latter round or rotating it, the light will be transmitted in
certain positions only, being partially or entirely arrested in others,
so that the field appears black. And, on careful examination, it will be
noticed that the change from black to white occurs at each quarter of a
rotation, being twice black and twice white in an entire rotation, the
changes occurring alternately. The same phenomena may also be exhibited
by substituting two Nicol’s prisms for the tourmalines.
Again, if we take a natural crystal of calcareous spar, and paste upon
one side of it a piece of black paper with a small hole in the middle,
on holding the crystal to the light or over a piece of white paper, with
the covered side next the light, two holes or two images of the hole
will be seen; and if the crystal without the paper be placed over some
print, the print will appear double. Hence the light passing through the
hole is twice or doubly refracted, one ray following the ordinary law of
refraction, while the other follows a different law, being retarded and
pursuing a longer course; and so the two rays are called respectively
the ordinary and extraordinary ray. And on viewing these through a
tourmaline or a Nicol’s prism, as in the experiment with the two
tourmalines, the images will become alternately visible and invisible,
just as was then the case with the entire mass of light.
The light which has undergone this singular change is said to be
polarized, because the rays appear to have acquired poles or sides. In
the above experiments the lower prism or tourmaline is called the
polarizer, because it polarizes the light, and the upper is called the
analyzer, because it analyzes or tests the light altered by the former.
An idea of the cause of this change may be obtained by reference to the
undulatory theory of light. Ordinary light consists of waves or
undulations taking place in planes at right angles to each other, or in
all planes; while in polarized light the undulations are all in one
plane or in parallel planes. This may perhaps be understood by
considering that books in a book-case are situated in parallel planes,
the shelves being in planes at right angles to the former. And by
imagining in polarizing substances the existence of some structure
acting like a grating, a notion can be obtained how the rays in the
different planes may be transmitted or intercepted. If the grating be so
placed that the bars (representing the planes of polarization) are
perpendicular, the books can pass between them; while if the grating be
turned round a quarter of a circle, they will become transverse, and the
books cannot pass, while the shelves could do so. Carrying on this
analogy, the tourmaline or Nicol’s prism polarizes the light by
transmitting only those rays whose undulations are in planes parallel to
the bars; while the analyzer allows these undulations to pass through it
when the direction of the planes coincides with that of the bars, but
interrupts them when their direction is at right angles to the bars. And
it is evident that the planes of polarization of the ordinary and
extraordinary rays are opposite, from the opposite action of the
analyzer upon them.
The power of doubly refracting and polarizing is not possessed by all
crystalline bodies, but only those belonging to other than the cubic
system; crystals belonging to this system neither doubly refract nor
polarize light. In all doubly refracting crystals there are one or more
lines or directions in which the light is not doubly refracted. These
are called the optic axes, and sometimes they coincide with the
geometric axis of the crystals, at others they do not; and they may be
regarded as positions or directions of equilibrium of certain molecular
forces existing within the crystal, which, acting in opposition,
neutralize each other.
If light, polarized by the polarizer, be transmitted through thin doubly
refracting crystals, and analyzed by the analyzer, splendid colours will
become visible; and on rotating separately either the polarizer or the
analyzer, at each quarter rotation the colours will change, being
complementary to those at first visible, or such as are requisite with
the first to make white light. We have seen (fig. 19) that white light
consists of seven coloured rays, or of three primary colours--red,
yellow, and blue, which, by superposition, form the others; and thus red
is complementary to green, which consists of blue and yellow, the two
sets of complementary colours appearing and vanishing as the light and
darkness did when the crystals were not used.
These colours are produced by interference. The compound rays of white
light (fig. 30 _l_) passing through the polarizer (_t_) are all
polarized in one plane; the crystal (_d_) depolarizes this light, _i.
e._ doubly refracts and resolves it into two sets of rays polarized in
planes at right angles to each other, forming the ordinary, O, and the
extraordinary ray, E. Each of these two sets of rays is resolved by the
analyzer (_s_) into two other sets, polarized in planes at right angles
to each other; so that in all there are four sets, two in one plane and
two in the other; and, the primary rays of the two sets in each plane
being in different stages or phases of undulation, in consequence of the
retardation of the extraordinary rays, the undulations of certain
coloured rays check and annihilate each other, while the remainder or
complementary conspire and pursue their course, producing the appearance
of colour, this effect being reversed at each quarter-revolution of the
analyzer.
An idea of what is meant by phases of undulation may be obtained by
reference to fig. 29, in which the undulations, _a_, _b_, are in similar
states or phases, and so conspire in action, while the wave _c_ is in a
different phase and half an undulation behind the others; hence it would
check or interfere with either of the other waves (_a_, _b_), the
etherial molecules of the two, which vibrate perpendicularly or at right
angles to the direction of the wave, acting to the same extent and in
opposite directions.
In fig. 30 the analyzer is represented as composed of a natural crystal
(rhomb) of calcareous spar, which transmits both sets of rays; but in
the ordinary analyzer or Nicol’s prism--which is made by dividing a
rhomb through the obtuse angles into two wedge-shaped pieces and
cementing them together again with balsam, only one set of rays is
transmitted at each quarter-revolution, the other being refracted out of
the field. In the case of the tourmaline, one of the sets of rays is
absorbed; so that the tourmaline, like the Nicol’s prism, is
single-imaged.
Thus the colours produced by polarization are the same as those of the
spectrum, but separated in a different way, both arising from the
elementary coloured rays of the compound white light. For while the
spectral rays are separated by dispersive refraction, the polarized
coloured rays are separated by the interference and annihilation of some
rays, the remainder passing on to produce the colours.
When the position of the depolarizing crystal is such that the plane of
the polarized light coincides with the direction of the optic axis or
axes, the light is not doubly refracted nor polarized in certain parts;
hence these parts appear white if the plane of the polarizer and
analyzer coincide, and black if they be crossed. A crystal cut at right
angles to its optic axis, with its length directed towards the polarizer
and analyzer, is in this position, and it exhibits alternately a black
and white cross, with one or two sets of concentric rings of
complementary colours at each quarter of a rotation.
To prepare crystals for examination by polarized light, a little Epsom
salt, nitre, or borax should be dissolved in water, a drop placed upon a
slide, and dried at a gentle heat. The crystals should then be mounted
in balsam, and viewed as transparent objects.
To see the cross and rings, the crystals should be sawn or cut across
transversely, the ends being polished on a strained piece of silk
moistened with water, and the sections mounted in balsam.
The property of doubly refracting and polarizing light is not confined
to crystalline substances, being also possessed by many organic bodies,
for the details of which I must refer to the article Polarization in the
‘Micrographic Dictionary.’
INDEX.
Abdomen, 129.
Aberration, chromatic, 173.
Aberration, spherical, 172.
Acalephæ, 153.
Acarus, 131.
Accumbent, 44.
Achromatic, 178.
Achromatic condenser, 5.
Achromatism, 177.
Acicular, 24.
Acotyledons, 43.
Acrocarpi, 55.
Acrostalagmus, 111.
Actinocyclus, 80.
Actinophrys, 156.
Adapter, 2.
Adulteration, 23.
Æcidium, 100.
Agaricus, 97.
Albumen, of seeds, 43.
----, of eggs, 113.
Algæ, 64.
Alyscum, 161.
Amœba, 155.
Amphileptus, 162.
Angiocarpi, 94.
Angle of vision, 175.
Anguillula, 152.
Angular, 23.
Animal tissues, 113.
Ankistrodesmus, 74.
Annular vessels, 27.
Annulus, of Ferns, 50.
----, of Fungi, 98.
----, of Mosses, 55.
Anoplura, 135.
Ant, 143.
Antennæ, of Entomostraca, 127.
Antennaria, 111.
Anthers, 37, 39.
Antheridia, of Algæ, 65.
----, of Ferns, 52.
Antheridia, of Mosses, 61.
Anthophysa, 160.
Antlia, 145.
Apex, 37.
Aphis, 146.
Apiculate, 101.
Aplanatic, 177.
Aplanatism, 176.
Apothecia, 92.
Apple-pulp, 19.
Apple, skin of, 41.
Arachnida, 130.
Arcella, 156.
Archegonia, of Ferns, 52.
----, of Mosses, 62.
Arcuate, 78.
Areolæ, 116.
Areolar tissue, 116.
Arrow-root, 23.
Arteries, 114.
Articulata, 127.
Asci, of Fungi, 97.
----, of Lichens, 92.
Ascomycetes, 107.
Ash, 25.
Aspergillus, 106.
Aspidium, 50.
Astasia, 160.
Atropos, 145.
Bacterium, 86.
Balsam, garden-, 25.
----, Canada, 12.
Bark, 36.
Basidia, 98.
Bast, 26.
Batrachospermeæ, 72.
Batrachospermum, 72.
Beards of Oyster, 125.
Bee, 143.
Beetles, 146.
Bermuda deposit, 81.
Bipinnatifid, 50.
Bivalve, 128.
Blood, 114.
----, of Birds, 119.
----, of Fishes, 121.
----, of Reptiles, 120.
Blood-corpuscles, 111.
Blood-worm (Chironomus), 143.
Blue-bottle, 137.
Blue mould, 106.
Bone, of Birds, 119.
----, of Reptiles, 120.
----, of Mammals, 114.
Bordered dots, 28.
Bothrenchyma, 28.
Botrytis, 105.
Branchiæ, 125.
Bread, 23.
Bryozoa, 125.
Bryum, 59.
Buds, 47.
Bug, 145.
Bull’s-eye, 6.
Bunt, 102.
Butterflies, 144.
Cabbage-butterfly, 145.
Calicium, 94.
Calyptra, 55.
Calyx, 37.
Calcareous, 66.
Camera lucida, 18.
Campylodiscus, 78.
Canaliculi, 115.
Cancelli, 114.
Capillaries, 114.
Capillary, 69.
Carapace, 127.
Caraway, 41.
Carpels, 41.
Cartilage, 115.
Caterpillars, 145.
Cell-contents, 21.
Cell-formation, 29.
Cells, 19.
Cell-wall, 19.
Cellular tissue, animal, 116.
---- tissue of plants, 20.
Cement, 16.
Cephalothorax, 130.
Ceramiaceæ, 69.
Ceramidia, 66.
Ceramium nodosum, 69.
---- rubrum, 70.
Cercomonas, 159.
Chætomium, 110.
Chætophoraceæ, 71.
Chara, 88.
Cheese-mite, 131.
Chelate, 131.
Chickweed, 37.
----, seeds of, 42.
Chilodon, 162.
Chin of Insects, 147.
Chironomus, 143.
Chitine, 136.
Chloride of calcium, 15.
Chlorococcum, 87.
Chlorophyll, 21.
Chrysalis, 141.
Chrysanthemum, 25.
Cilia, 52, 157.
Circulation in Plants, 31.
Cladonia, 93.
Cladophora, 71.
Classification, 164.
Clavarini, 99.
Clavate, 147.
Closterium, 74.
Coarse movement, 2.
Coccidia, 69.
Coccinella, 147.
Cocconeis, 79.
Cocoa-nut fibre, 26.
Cœlenterata, 153.
Coleochæte, 72.
Coleoptera, 146.
Coleps, 163.
Collomia, seeds of, 42.
Colpoda, 161.
Columella, of Mosses, 62.
----, of Mucor, 111.
Complementary colours, 183.
Compound animals, 126.
Concentric, 114.
Conceptacles, of Algæ, 65.
Conduplicate, 44.
Conferva bombycina, 71.
---- floccosa, 71.
Confervaceæ, 70.
Confervoideæ, 70.
Conidia, 100.
Coniferæ, 26.
Coniomycetes, 99.
Conjugation, 73.
Continuous, 103.
Convolvulus, pollen of, 39.
Coral, 153.
Corallina, 66.
Corallinaceæ, 66.
Corallines, 66.
Coremium, 106.
Corolla, 37.
Coscinodiscus, 81.
Cotton, 118.
Cotton-plant, 33.
Cotyledons, 43, 44.
Covers, 6.
Coxa, 139.
Crenate, 50.
Crocus, pollen-tubes of, 40.
Cruciferæ, 44.
Crustacea, 127.
Crustaceous, 91.
Cryptogamic plants, 48.
---- ----, seeds of, 43.
Ctenoid, 121.
Cucumber, 42.
Culex, 141.
Culm, 34.
Cuneate, 79.
Cup-moss, 93.
Curculionidæ, 149.
Cycloid, 121.
Cyclops, 128.
Cypris, 128.
Dacrymyces, 99.
Daphnia, 129.
Dasya, 68.
Day-flies, 145.
Deal, 26.
Delesseria, 69.
Dematiei, 104.
Desmidiaceæ, 73.
Diamond-beetles, 149.
Diaphragm, 3.
Diatoma, 78.
Diatomaceæ, 75.
----, fossil, 82.
----, markings on, 81.
----, preparation of, 82.
Dichotomous, 67.
Dicotyledons, leaves of, 35.
----, seeds of, 43.
----, stems of, 35.
Dicranum, 57.
Differentiation, 47.
Dimidiate, 55.
Dinobryina, 160.
Dinobryon, 160.
Diœcious, 38.
Dipping-tubes, 4.
Diptera, 137.
Dispersion, 178.
Division, spontaneous, 159.
Dothidea, 108.
Dragon-flies, 145.
Draparnaldia, 71.
Dry rot, 98.
Ducts, 27.
Dung-beetle, 132.
Dytiscus, 148.
Eccremocarpus, seeds of, 42.
Echinodermata, 153.
Echinus, 153.
Elbowed, 149.
Elder, 25.
Elements, vegetable, 19.
Elytra, 147.
Emarginate, 146.
Embryo, 42.
Embryo-sac, 46.
Embryonal vesicles, 46.
Enchelys, 161.
Encysting, 159.
Endogenous, 29.
Endogens, leaves of, 35.
----, seeds of, 44.
----, stems of, 44.
Enteromorpha, 87.
Entomostraca, 127.
Entozoa, 151.
Ephemera, 145.
Epidermis, animal, 117.
----, vegetable, 31.
Epithemia, 77.
Equisetum, 34.
Erector, 5.
Erysiphe, 110.
Euglena, 160.
Excipulum, 92.
Excurrent, 57.
Exogenous, 29.
Exogens, leaves of, 35.
----, seeds of, 43.
----, stems of, 35.
Eye-pieces, 4, 179.
Eyes, compound, 138.
Facets, 138.
Favellæ, 70.
Feathers, 119.
Feathery, 127.
Femur, 139.
Ferns, 49.
----, reproduction of, 51.
Fertilization, 45.
Fibro-cellular tissue, 25.
Fibro-vascular tissue, 31.
Field, 7.
Filament, 37.
----, flagelliform, 159.
Fine movement, 2.
Fish-crystals, 122.
Flagelliform, 159.
Flax, 26, 118.
Flea, 136.
Flocci, 103.
Florideæ, 66.
Floscularia, 151.
Flowers, 37.
Flustra, 126.
Fly, 138.
Fly-blows, 139.
Focus, 171.
Foot-jaws, 127.
Foramen, 45.
Foramina, 114.
Foraminifera, 156.
Forceps, 4.
Forked veins, 50.
Fovilla, 160.
Fragilaria, 78.
Fronds, 49.
Fruit, 40.
Frustules, 75.
Fucoideæ, 64.
Fucus, 65.
Fulcra, 110.
Funaria, 58.
Fungi, 96.
Funiculus, 45.
Fusiform, 110.
Galls, 111.
Gamasus, 132.
Gasteromycetes, 99.
Gemmation, 159.
Gentles, 140.
Geranium, leaf of, 21.
----, petals of, 20, 38.
Germinate, 43.
Gills, of Fungi, 97.
----, of Mollusca, 125.
Glands, 40.
Glandular, 31.
---- tissue, 26.
Glœocapsa, 88.
Glumes, 37.
Glycerine, 14.
Gnats, 141.
Gomphonema, 79.
Gonidia, of Algæ, 71.
----, of Lichens, 92.
Gonium pectorale, 83.
---- tranquillum, 84.
Grape-Fungus, 105.
Graphis, 94.
Green fly, 146.
Gristle, 115.
Groundsel, 33.
Guard-cells, 34.
Gymnocarpi, 94.
Gymnostomum, 56.
Gyrosigma angulatum, 80.
---- attenuatum, 79.
Hairs, 31.
----, animal, 117.
Hart’s-tongue Fern, 51.
Harvest-bug, 132.
Haversian canals, 114.
Helvellacei, 107.
Hemiptera, 145.
Hemp, 26.
Hermaphrodite, 38.
Heteromita, 160.
Hilum, 22.
Hoop, 76.
Humble Bee, 144.
Hundred-legs, 133.
Hyalotheca, 74.
Hydnei, 99.
Hydra, 153.
Hymenium, 98.
Hymenomycetes, 97.
Hymenoptera, 143.
Hyphomycetes, 103.
Hypnea, 69.
Hypnum, 60.
Hysterium, 108.
Illumination, 8.
Imago, 141.
Imbricate, 56.
Incumbent, 44.
Indusium, 51.
Inflated, 110.
Infusoria, 157.
Injection, 116.
Innovations, 55.
Insects, 133.
Intercellular passages, 20.
Intercellular spaces, 20.
Iodine, 22.
Isaria, 104.
Jania, 67.
Jaws, of Insects, 147.
Joints, of Insects, 136.
Jute, 26.
Knife, 4.
Labels, 17.
Labial palpi, 134.
Labium, 147.
Labrum, 147.
Lacunæ, 115.
Lady-bird, 147.
Lanceolate, 78.
Leaves, 31.
Legs, of insects, 139.
Lenses, 170.
Lentil, 23.
Lepidoptera, 144.
Lepisma, 134.
Liber, 26, 35.
Lice, 135.
Lichens, 91.
Lieberkuhn, 7.
Light, 167.
Lime, 25.
Lips, of insects, 147.
Lirellæ, 94.
Lithobius, 133.
Lithocystis, 67.
Live-box, 4.
London pride, anthers of, 39.
Longitudinal, 30.
Lunate, 147.
Lyngbya, 85.
Magnification, 175.
Magnifying power, 17.
Mammalia, 113.
Mandibles, of insects, 147.
----, of Spiders, 130.
Mantle, 123.
Marrow, 114.
Matrix, 91, 96.
Maxillæ, 147.
Medium, 168.
Medulla, 35.
Medullary sheath, 35.
Melanosporeæ, 64.
Melobesia, 67.
Melosira, 79.
Membrane, 31.
Membranipora, 126.
Mentum, 147.
Merulius, 98.
Micrasterias, 74.
Micropyle, of seeds, 46.
Microscope, 1.
----, simple, 5.
Midrib, 50.
Mignonette, seeds of, 42.
Mildew, 101.
Mirror, 2, 3.
Mitriform, 55.
Mollusca, 122.
Monadina, 159.
Monas, 159.
Moniliform, 31.
Monocotyledons, leaves of, 35.
----, seeds of, 43.
----, stems of, 36.
Monœcious, 38.
Mosses, 54.
Moths, 144.
Mould of paste, 111.
Moulds, 96.
Mounting, 10.
Mouth of insects, 147.
Mucedines, 105.
Mucor, 111.
Muscles, 115.
Mushroom, 97.
Mustard and cress, 43.
----, cotyledons of, 44.
Mycelium, 96.
Myriapoda, 133.
Nacre, 123.
Nassula, 162.
Needles, mounted, 4.
Nemaspora, 100.
Nerves, of leaves, 31.
Neuroptera, 145.
Newt, 120.
Nicol’s prism, 6, 184.
Nitella, 88.
Nitzschia, 78.
Nodding, 59.
Nodule, 79.
Nostoc, 86.
Nucleolus, 21.
Nucleus, 21.
----, of Infusoria, 159.
----, of ovule, 45.
Oak-apple, 111.
Oat, 23.
Object-glasses, 2.
Oblique, 30.
---- light, 9.
Obovate, 57.
Obtuse, 50.
Ocelli, of Spiders, 130.
Oidium, 105.
Opegrapha, 94.
Operculum, 55.
Optic axes, 183.
Orange, rind of, 40.
Organs, vegetable, 31.
Oscillatoria autumnalis, 84.
---- nigra, 85.
Oscillatoriaceæ. 84.
Ovary, of animals, 137.
---- of plants, 37, 40.
Ovate, 56.
Ovigerous vesicles, 154.
Ovisacs, 154.
Ovules, 45.
Ovum, 113.
Oxytricha, 161.
Oyster-shell, 123.
Paleæ, 37.
Palmella, 88.
Palmellaceæ, 87.
Palpi, of insects, 147.
Papilla, 39.
Papillæ, of skin, 116.
Papillose, 39.
Paramecium, 161.
Paraphyses, 61.
Parenchyma, 20.
Parmelia, 91.
Pear, 29.
Pearls, 123.
Pectinate, 69.
Pediastrum, 74.
Pedicels, 103.
Pencil, 128.
Penicillium, 106.
Perianth, 37.
Pericarp, 40.
Perichætial, 60.
Peridiola, 110.
Peridium, 99, 100.
Perigonial, 61.
Perispore, 65.
Peristome, 55.
Perithecium, 107.
Petals, 37, 38.
Peziza, 107.
Phases, 184.
Phragmidium, 101.
Physarum, 99.
Physomycetes, 110.
Pigment, 117.
Pileus, 97.
Pinnæ, 51.
Pinnate, 51.
Pinnatifid, 50.
Pinnularia, 79.
Pinnules, 51.
Pistil, 37.
Placenta, 45.
Pleurenchyma, 26.
Pleurocarpi, 55.
Pleurosigma, 79.
Plocamium, 69.
Plumose, 127.
Plum-stone, 29.
Plumule, 43.
Podetia, 93.
Podophrya, 162.
Podura, 134.
Polariscope, 6.
Polarization of light, 180.
Pollen, 37.
Pollen-granules, 39.
Pollen-tubes, 40.
Polycystina, 156.
Polypi, 153.
Polypidom, 125.
Polypodiaceæ, 50.
Polypodium, 50.
Polyporei, 99.
Polysiphonia, 68.
Polytrichum, 58.
Polyzoa, 126.
Polyzoary, 126.
Poppy, seeds of, 42.
Pore, 109.
Pores, in wood, 28.
Porous cells, 25.
Potato, 22.
Potato-fungus, 105.
Primine, 45.
Primordial utricle, 21.
Primrose, pollen of, 39.
Prismatic, 24.
Proboscis, 146.
Prolegs, 145.
Prosenchyma, 25.
Prothallium, 51.
Protoplasm, 21.
Protozoa, 155.
Pterodina, 151.
Puccinia, 101.
Puff-balls, 99.
Pulex, 135.
Pulvilli, 139.
Puncta, 79.
Pupa, 141.
Quadrate, 147.
Racodium, 111.
Radiata, 153.
Radicle, 43.
Ramenta, 49.
Raphides, 24.
Receptacles, of Algæ, 65.
----, of secretion, 40.
Red Spider, 132.
Reflexion, 169.
Refraction, 168.
Reindeer moss, 93.
Reproductive organs, 48.
Reptiles, 120.
Resting spores, 83.
Reticulated, 54, 129.
---- vessel, 27.
Rhabdonema, 80.
Rhinotrichum, 107.
Rhizome, 49.
Rhizopoda, 155.
Rhodomelaceæ, 67.
Rhodospermeæ, 66.
Rhodymeniaceæ, 69.
Rhubarb, 25.
Rice, 23.
Richmond earth, 82.
Rings, annual, 36.
Roe of fishes, 122.
Rootlets, 36.
Roots, 36.
Rostrate, 56.
Rostrum, 131.
Rotating disk, 6.
Rotation, 31.
Rotatoria, 149.
Rotifer, 150.
Rotifera, 149.
Sacculi, gastric, 158.
San Fiori deposit, 82.
Sarcode, 155.
Scalariform ducts, 49.
Scales, of fishes, 121.
----, of Podura, 134.
----, of insects, 144.
Scar of seeds, 46.
Scenedesmus, 74.
Schizogonium, 86.
Scolopendrium, 51.
Sea-weeds, 64.
Sections, 30.
Secundine, 45.
Seeds, 42.
Selenite, 24.
Sepals, 37.
Septa, 99.
Septate, 100.
Serrate, 51.
Sertularia, 154.
Sessile, 54.
Setaceous, 133.
Setæ, 129.
---- of Infusoria, 158.
Shell, 122.
Side condenser, 6.
Silk, 118.
Simple microscope, 5.
Sines, 168.
Siphonaceæ, 84.
Skin, 116.
Slides, 6.
Smut, 102.
Snapdragon, petals of, 39.
Snowberry, 42.
Sori, of Ferns, 50.
Spawn, of Fungi, 97.
Spectrum, 173.
Spermatia, 93.
Spermatozoa, of Algæ, 65.
----, of Ferns, 52.
----, of Fishes, 122.
----, of Mosses, 61.
Spermogonia, of Fungi, 101.
----, of Lichens, 92.
Sphæria, 109.
---- fragiformis, 104.
Sphagnum, 56.
Spicula, 157.
Spiders, 130.
Spiderwort, 31.
Spinnerets, 131.
Spiracles, 134.
Spiral cells, 25.
Spiral vessels, 27.
Spirit-lamp, 5.
Spirogyra, 72.
Spirulina, 85.
Sponges, 156.
Spongidæ, 156.
Sporangium, of Ferns, 54.
Spores, 50.
Sporocybe, 104.
Sporophylles, 69.
Stage, 2.
Stamens, 37.
Starch, 22.
Starfishes, 153.
Stellate, 31.
Stems, 35.
Stentor, 163.
Sterigmata, 98.
Steropus, 146.
Stichidia, 68.
Stigma, 37.
Stilbacei, 103.
Sting, of Wasp, 144.
Stinging-organs, 154.
Stings, 33.
Stipes, 76, 97.
Stomata, 34.
Stomoxys, 141.
Striated, 33.
Stroma, 108.
Struma, 58.
Style of plants, 37.
Stylospores, 100.
Suctorial, 135.
Suture, 73, 76.
Synedra, 78.
Synura, 83.
Tabellaria, 79.
Tabular, 83.
Tangential, 30.
Tape-worm, 152.
Tarsus, 139.
Tentacles, 124, 153.
Testa, 42.
Tetraspores, 167.
Thalamium, 92.
Thallus, 91.
Thecæ, 50.
Thorax, 129.
Thread-worm, 152.
Tibia, 139.
Tissues, vegetable, 19.
Toad-stools, 96.
Tongue, of Mollusca, 123.
Tortula, 57.
Torula, 100.
Tourmaline, 181.
Tous les mois, 24.
Trabeculæ, 59.
Tracheæ, 134.
Trachelomonas, 160.
Tradescantia, 31.
Transverse, 148.
---- section, 30.
Tremellini, 99.
Trichothecium, 105.
Triton, 120.
Trochanter, 139.
Trombidium, 132.
Truffle, 108.
Truncated, 27.
Tuberacei, 108.
Tubercularia, 103.
Tuberculate, 101.
Tubular cells, 25.
Tufts, 25.
Turnip-fly, 146.
Turpentine, 27.
Ulothrix, 86.
Ulva, 87.
Ulvaceæ, 86.
Umbelliferæ, 41.
Uncorrected lens, 178.
Uniseptate, 101.
Uredo, 102.
---- candida, 102.
---- caries, 102.
---- linearis, 109.
---- rubigo, 109.
---- segetum, 102.
Vaginicola, 163.
Vaginule, 62.
Valve, 76.
Vascular tissue, 27.
Vaucheria, 84.
Vegetable elements, 19.
---- organs, 31.
Veil, 98.
Veins, of animals, 114.
----, of leaves, 31, 35.
Velum, 98.
Vertebra, 113.
Vertebrata, 113.
Vesicles, contractile, 159.
----, embryonal, 46.
Vessels, 27.
Vibrio spirillum, 85.
Vision, 174.
Vittæ, of seeds, 40.
Volva, 98.
Volvocineæ, 83.
Volvox, 83.
Vorticella, 162.
Wallflower, anthers of, 39.
Wasp, 143.
Water-beetle, 148.
Web, spider’s, 131.
Wheat, cotyledon of, 45.
Wheat-flour, 23.
Wheel-animalcules, 150.
Whelk, 123.
Wine-cellar Fungus, 111.
Winged seeds, 42.
Winter ova, 128.
Wood-cells, 25.
Wood-louse, 127.
Woody fibre, 25.
---- tissue, 25.
Wool, 118.
Xanthidia, 75.
Yeast, 106.
Zygnema, 72.
Zygnemaceæ, 2.
Zoospores, 71.
TAYLOR AND FRANCIS, PRINTERS, RED LION COURT, FLEET STREET.