Many forms of heliostats have been devised, the earliest having been
described by Wilhelm Jacob s' Gravesande in the 3rd edition of his
_Physices elementa_ (1742). One of the simplest consists of a plane
mirror rigidly connected with a revolving axis so that the angle
between the normal to the mirror and the axis of the instrument equals
half the sun's polar distance, the mirror being adjusted so that the
normal has the same right ascension as the sun. It is easily seen that
if the mirror be rotated at the same angular velocity as the sun the
right ascensions will remain equal throughout the day, and therefore
this device reflects the rays in the direction of the earth's axis; a
second fixed mirror reflects them in any other fixed direction.
Foucault's heliostat reflects the rays horizontally in any required
direction. The principle of the apparatus may be explained by
reference to fig. 1. The axis of rotation AB bears a rigidly attached
rod DBC inclined to it at an angle equal to the sun's polar distance.
By adjusting the right ascension of the plane ABC and rotating the
axis with the angular velocity of the sun, it follows that BC will be
the direction of the solar rays throughout the day. X is the mirror
rotating about the point E, and placed so that (if EB is the
horizontal direction in which the rays are to be reflected) (1) the
normal CE to the mirror is jointed to BC at C and is equal in length
to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and
in the plane of, the mirror. Since CE equals BE these directions are
equally inclined to, and coplanar with, the normal to the mirror.
Hence light incident along the direction BC will be reflected along
CE. Silbermann's heliostat reflects the rays in any direction. The
principle may be explained by means of fig. 2. AB is the axis of
rotation, BC an adjustable rod as in Foucault's construction, and BD
is another rod which can be set to the direction in which the rays are
to be reflected. The rods BC and DB carry two small rods EF, GF
jointed at F; at this joint there is a pin which slides in a slot on
the rod BH, which is normal to the mirror X. The rods EF, GF are such
that BEFG is a rhombus. It is easy to show that rays falling on the
mirror in the direction BC will be reflected along BD. One
construction of the instrument, described in Jamin's _Cours de
physique_, is shown in fig. 3. The mirror mm is attached to the
framework _pafe_, the members of which are parallel to the incident
and reflected rays SO, OR, and the diagonal pf is perpendicular to the
mirror. The framework is attached to two independent circular arcs Cs
and rr' having their centres at O and provided with clamps D and A on
the axis F of the instrument. The arc Cs is graduated, and is set so
that the angle COD equals the complement of the sun's declination.
This can be effected (after setting the axis) by rotating Cs until a
needle indicates true time on the hour dial B. The arc rr' is set so
as to reflect the rays in the required direction. The axis F of the
instrument is set at an angle equal to the latitude of the place of
observation and in the meridian by means of the screw K, and rotated
by clockwork contained in the barrel H. The setting in the meridian is
effected by turning the instrument after setting for latitude until a
pin-hole aperture s and a small screen P, placed so that Ps is
parallel to CO, are in a line with the sun.
Many other forms of heliostats have been designed, the chief
difference consisting in the mechanical devices for maintaining the
constant direction of the reflecting ray. One of the most important
applications of the heliostat is as an adjunct to the newer forms of
horizontal telescopes (q.v.) and in conjunction with spectroscopic
telescopes in observations of eclipses.
HELIOTROPE, or TURNSOLE, _Heliotropium_ (Gr. [Greek: heliotropion], i.e.
a plant which follows the sun with its flowers or leaves, or, according
to Theophrastus (_Hist, plant_, vii. 15), which flowers at the summer
solstice), a genus of usually more or less hairy herbs or undershrubs of
the tribe _Heliotropieae_ of the natural order Boraginaceae, having
alternate, rarely almost opposite leaves; small white, lilac or blue
flowers, in terminal or lateral one-sided simple or once or twice forked
spikes, with a calyx of five deeply divided segments, a salver-shaped,
hypogynous, 5-lobed corolla, and entire 4-celled ovary; fruit 2- to
4-sulcate or lobed, at length separable into four 1-seeded nutlets or
into two hard 2-celled carpels. The genus contains 220 species
indigenous in the temperate and warmer parts of both hemispheres. A few
species are natives of Europe, as _H. europaeum_, which is also a
naturalized species in the southern parts of North America.
[Illustration: _Heliotropium suaveolens._]
The common heliotrope of English hothouses, _H. peruvianum_, popularly
known as "cherry-pie," is on account of the delicious odour of its
flowers a great favourite with florists. It was introduced into Europe
by the younger Jussieu, who sent seed of it from Peru to the royal
garden at Paris. About the year 1757 it was grown in England by Philip
Miller from seed obtained from St Germains. _H. corymbosum_ (also a
native of Peru), which was grown in Hammersmith nurseries as early as
1812, has larger but less fragant flowers than _H. peruvianum_. The
species commonly grown in Russian gardens is _H. suaveolens_, which has
white, highly fragrant flowers.
Heliotropes may be propagated either from seed, or, as commonly, by
means of cuttings of young growths taken an inch or two in length.
Cuttings when sufficiently ripened, are struck in spring or during the
summer months; when rooted they should be potted singly into small pots,
using as a compost fibry loam, sandy peat and well-decomposed stable
manure from an old hotbed. The plants soon require to be shifted into a
pot a size larger. To secure early-flowering plants, cuttings should be
struck in August, potted off before winter sets in, and kept in a warm
greenhouse. In the spring larger pots should be given, and the plants
shortened back to make them bushy. They require frequent shiftings
during the summer, to induce them to bloom freely.
The heliotrope makes an elegant standard. The plants must in this case
be allowed to send up a central shoot, and all the side growths must be
pinched off until the necessary height is reached, when the shoot must
be stopped and lateral growths will be produced to form the head. During
winter they should be kept somewhat dry, and in spring the ball of soil
should be reduced and the plants repotted, the shoots being slightly
pruned, so as to maintain a symmetrical head. When they are planted out
against the walls and pillars of the greenhouse or conservatory an
abundance of highly perfumed blossoms will be supplied all the year
round. From the end of May till October heliotropes are excellent for
massing in beds in the open air by themselves or with other plants. Many
florists' varieties of the common heliotrope are known in cultivation.
Pliny (_Nat. hist._ xxii. 29) distinguishes two kinds of "heliotropium,"
the _tricoccum_, and a somewhat taller plant, the _helioscopium_; the
former, it has been supposed, is _Croton tinctorium_, and the latter the
[Greek: heliotropion mikron] of Dioscorides or _Heliotropium europaeum_.
The helioscopium, according to Pliny, was variously employed in
medicine; thus the juice of the leaves with salt served for the removal
of warts, whence the term _herba verrucaria_ applied to the plant. What,
from the perfume of its flowers, is sometimes called winter heliotrope,
is the fragrant butterbur, or sweet-scented coltsfoot, _Petasites_
(_Tussilago_) _fragrans_, a perennial Composite plant.
HELIOTROPE, in mineralogy, is the mineral commonly called "bloodstone"
(q.v.), and sometimes termed girasol--a name applied also to fire-opal.
The name, like those of many ancient names of minerals, seems to have
had a fanciful origin. According to Pliny the stone was so called
because when thrown into the water it turned the sun's light falling
upon it into a reflection like that of blood.
HELIOZOA, in zoology, a group of the Sarcodina (q.v.) so named by E.
Haeckel, 1866. They are characterized by the radiate pseudopods, finely
tapering at the apex, springing abruptly from the superficial
protoplasm, containing a denser, rather permanent axial rod (figs. 1
(1), 2 (2)); protoplasm without a clear ectoplasm or pellicle, often
frothy with large vacuoles, like the alveoli of Radiolaria; nucleus 1 or
numerous; skeleton absent, gelatinous or of separate siliceous fibres,
plates or spicules, rarely complete and latticed; reproduction by simple
fission or by brood-formation, often syngamous; form usually nearly
spherical, rarely changing slowly. This group was formerly included with
the Rhizopoda; but was separated from it by Haeckel on account of the
character of its pseudopods, and its general adaptation to a semipelagic
existence correlated with the frothy cytoplasm (fig. 1 (1)).
_Actinophrys sol_ and _Actinosphaerium eichhornii_ (fig. 2), known as
sun animalcules to the older microscopists, float freely in stagnant or
slow-flowing waters, and _Myriophrys_ is able by an investment of long
flagelliform cilia to swim freely. The majority, however, lurk among
confervae or the light debris of the bottom ooze; and come under the
head of "sapropelic" rather than pelagic organisms. The body is usually
of constant spherical form in relation to the floating habit.
_Nuclearia_, however, shows amoeboid changes of general outline. The
pseudopods are retractile, the axial filament being absorbed as the
filament grows shorter and thicker and disappearing when the pseudopod
merges into the ectoplasm, to be reformed at the same time with the
pseudopod. There is often a distinction, clear, but never sharp, between
the richly vacuolate, almost frothy ectoplasm and the denser endoplasm.
One or more contractile vacuoles may protrude from the ectoplasm. The
endoplasm contains the nucleus or nuclei. The nucleus when single may be
central or excentric: in the latter case, the endoplasm contains a clear
central sphere ("centrosome") on which abut the axial filaments of the
pseudopods. The ectoplasm contains, in some species, constantly
(_Raphidiophrys viridis_) or occasionally (_Actinosphaerium_), green
cells belonging to the genera _Zoochlorella_ and _Sphaerocystis_, both
probably--the latter certainly--vegetative stages of a Chlamydomonad
(FLAGELLATA, q.v.) and of symbiotic significance.
[Illustration: FIG. 1.--Heliozoa. 1. _Actinophrys sol_, Ehrb. a,
food-particle lying in a large food-vacuole; b, deep-lying finely
granular protoplasm; c, axial filament of a pseudopodium extended
inwards to the nucleus; d, the central nucleus; e, contractile vacuole;
f, superficial much vacuolated protoplasm. 2. _Clathrulina elegans_,
Cienk. 3. _Heterophrys marina_, H. and L. a, nucleus; b, clearer
protoplasm surrounding the nucleus; c, the peculiar felted envelope. 4.
_Raphidiophrys pallida_, F. E. Schultze. a, food-particle; b,
contractile vacuole; c, the nucleus; d, central granule in which all the
axis-filaments of the pseudopodia meet. The tangentially disposed
spicules are seen arranged in masses on the surface. 5. _Acanthocystis
turfacea_, Carter. a, probably the central nucleus; b, clear protoplasm
around the nucleus; c, more superficial protoplasm with vacuoles and
chlorophyll corpuscles; d, coarser siliceous spicules; e, finer forked
siliceous spicules; f, finely granular layer of protoplasm. The long
pseudopodia reaching beyond the spicules are not lettered. 6.
Bi-flagellate "flagellula" of _Acanthocystis aculeata_. a, nucleus. 7.
Id. of _Clathrulina elegans_. a, nucleus; b, granules. 8. _Astrodisculus
ruber_, Greeff. a, red-coloured central sphere (? nucleus); b,
peripheral homogeneous envelope.]
The Heliozoa can move by rolling over on their extended pseudopods;
_Acanthocystis ludibunda_ traversing a path of as much as twenty times
its diameter in a minute, according to Penard. Several species (e.g.
_Raphidiophrys elegans_) remain associated by the union of their
pseudopods, whether into social aggregates (due to approximation) or
"colonies" due to lack of separation after fission, is not accurately
known. The multinuclear species _Actinosphaerium eichhornii_ (fig. 2),
normally apocytial (i.e. the nuclei divide repeatedly without division
of the cytoplasm), may increase in size by the fusion ("plastogamic")
of small individuals. If a large specimen be cut up or fragment itself
under irritation, the small ones so produced soon approach one another
and fuse completely.
[Illustration: FIG. 2.--Heliozoa. 1. _Actinosphaerium eichhornii_, Ehr.;
a, nuclei; b, deeper protoplasm with smaller vacuoles and numerous
nuclei; c, contractile vacuoles; d, peripheral protoplasm with larger
vacuoles. 2. A portion of the same specimen more highly magnified and
seen in optical section. a, Nuclei; b, deeper protoplasm (so-called
endosarc); d, peripheral protoplasm (so-called ectosarc); e, pseudopodia
showing the granular protoplasm streaming over the stiff axial filament:
f, food-particle in a good-vacuole. 3, 4. Nuclei of _Actinosphaerium_ in
the resting condition. 5-13. Successive stages in the division of a
nucleus of _Actinosphaerium_, showing fibrillation, and in 7 and 8
formation of an equatorial plate of chromatin substance (after Hertwig).
14. Cyst-phase of _Actinosphaerium eichhornii_, showing the protoplasm
divided into twelve chlamydospores, each of which has a siliceous coat;
a, nucleus of the spore; g, gelatinous wall of the cyst; h, siliceous
coat of the spore.]
_Reproduction._--Binary fission has been repeatedly observed; in some
cases one or both of the daughter cells may swim for a time as a
biflagellate zoospore (fig. 1 (6, 7)). The process may take place when
the cell is naked or after preliminary encystment. Budding has been
well studied in _Acanthocystis_; the cell nucleus divides repeatedly
and most of the daughter nuclei pass to the periphery, aggregate part
of the cytoplasm, and with it are constricted off as independent
cells; one nucleus remains central and the process may be repeated.
The detached bud may assume the typical character after a short
amoeboid (lobose) stage, sometimes preceded by rest, or it may develop
2 flagella and swim off (fig. 1 (6)).
Brood formation is only known here in relation to a syngamic process;
this is a sharp contrast to Proteomyxa (q.v.) where brood formation is
the commonest mode of reproduction, and plasmodium-formation, rare
indeed, is the nearest approach to syngamy observed. Indeed, if we
knew the life-history of all the species this difference in the life
cycle would be a convenient critical character.
Equal conjugation was demonstrated fully by F. Schaudinn in
_Actinophrys_; two individuals approach and enter into close contact,
and are surrounded by a common cyst wall. The nucleus of either male
divides; and one nucleus passes to the surface at either side, and is
budded off with a small portion of the cytoplasm as an abortive cell;
the two remaining nuclei which are "first cousins" in cellular
relationship now fuse, as is the case with the cytoplasts. The
resulting coupled cell or zygote divides into two, which again encyst.
_Actinosphaerium_ (fig. 2) shows a still more remarkable process,
fully studied by R. Hertwig. The large multinucleate animal withdraws
its pseudopods, its vacuoles disappear, it encysts and its nuclei
diminish in number to about 1/20th partly by fusion, 2 and 2, probably
by digestion of the majority. Within the primary cyst the body is now
resolved into nuclear cells, which again surround themselves with
secondary cysts. The cell in each secondary cyst divides (by
karyokinesis), and these sister cells, or rather their offspring, pair
in much the same way as the individual cells of _Actinophrys_--the
chief difference is that after the first division and budding off of a
rudimentary cell, a second division of the same character takes place,
with the formation of a second rudimentary cell, which is the niece of
the first, absolutely in the same way as the 1st and 2nd polar bodies
are formed in the maturation of the ovum in Metazoa. The actual
pairing cells are thus second cousins, great-granddaughters of the
original cell of the secondary cysts. Complete fusion now takes place
to form the coupled cell, which is now contracted and forms a
gelatinous wall within the siliceous secondary cyst wall (fig. 2
(14)), During a resting stage nuclear divisions occur and finally a
brood of young 1-nuclear _Actinosphaerium_ leave the cyst.
_Classification._
Aphrothoraca. Body naked. Actinophrys Ehrb. (fig. 1 (1)) (nucleate),
Actinosphaerium Stein plurinucleate (fig. 2 (1)), Camptonema
(plurinucleate) Schaud., Dimorpha Gruber (sometimes 2 flagellate).