Mi′cro-scope.
An optical instrument by which objects too small to be viewed by the naked eye may be seen and examined.
A
single or simple microscope is one by which the object is seen directly; it may consist of a single lens or of more than one.
In a
compound microscope two or more lenses are so arranged that the image formed by one is magnified by the others, and viewed as if it were the object itself.
In a
solar microscope a reflector and condenser are employed to direct the sun's rays on the object.
In a
lucernal microscope the rays of a lamp are similarly directed.
The magnifying power of glass balls was known in early times to the Chinese, Japanese, Assyrians, and Egyptians, and more lately among the Greeks and Romans.
The use of lenses for microscopes long preceded their application to telescopes.
Sir David Brewster exhibited one in 1852 which was made of rock crystal, and was found among the ruins of
Nineveh.
The refractive power of glass was known to Ptolemy, who gives a table of the deviation luminous rays experience when passing through glass under different angles of incidence.
Descartes discovered the law that the amount of this refraction is proportional to the sine of the angle of incidence, and from that time date all great discoveries in optics.
Glass balls, called burning spheres, were sold in
Athens before the Christian era.
Their magnifying power was mentioned by the
Roman philosopher
Seneca, and by
Porta of
Naples in his “Magia Naturalis,” published in 1590.
Alhazen, A. D. 1100, was the first to correct the popular error as to the nature of vision, showing that the rays came from the object and did not issue from the eye, as had been previously supposed.
He determined that the retina is the seat of vision, and that impressions made on it by light are conveyed from it along the optic nerve to the brain; he explained that the formation of the visual images on corresponding portions of the two retinas is the cause of our seeing single when we use both eyes; he showed that a ray of light entering the atmosphere obliquely is deflected and follows a curvilinear path, owing to the increasing density of the air as the rays travel towards the earth, and deduced
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from the data that the limit of the hight of the atmosphere is about 58 1/2 miles.
Huyghens states that Drebell had a microscope in 1621, and that he was the reputed inventor of it; the invention is also claimed by Fontana, a Neapolitan, in 1618.
Burrell asserts that the Jansens, father and son, made the first microscope and presented it to Prince Maurice and
Archduke Albert of
Austria.
The invention was, however, clearly anticipated by
Roger Bacon.
Spectacles were in use A. D. 1200.
The double microscope was invented by Farncelli in 1624.
Dr. Hooke (1635 – 1702) made microscopes of a sphere of glass, from 1/20 to 1/50 of an inch in diameter.
Having made and polished his lens, he placed it against a small hole in a thin piece of metal and fixed it with wax.
Leuwenhoeck's microscopes, 26 in number, which he presented to the Royal Society, have each a doubleconvex lens.
Their powers were from 40 to 160.
“Comes
Mr. Reeve, with a microscope and a scotoscope.
For the first I did give him £ 5 10
s., a great price, but a most curious bawble it is.” — Pepys's
Diary, 1664.
Stephen Gray's “poor-man's microscope,” 1696, was merely a drop of water suspended at the end of a wire or pin. A piece of perforated cardboard or sheet-metal affords a better means of holding the drop of water, whose rounded surfaces give it the properties of a lens.
By fusing in the flame of a spirit-lamp a small piece of glass contained within a ring of platinumwire, a lens may be formed.
A piece of the wire should project, so as to form a handle.
Plate-glass is the material commonly used for microscopic lenses.
Flint-glass, on account of its great dispersive power, is unsuited for the purpose.
Rock-crystal is an excellent material.
The diamond and other precious stones have been employed, but the advantage of their high refractive power is found to be counterbalanced by their color, reflective power, double refraction, and heterogeneous structure.
The crystalline lenses from the eyes of small fishes give a very perfect image of minute objects.
The lineal magnifying power of a single lens is equal to the distance of distinct vision divided by the focal length of the lens.
The object is brought a little within the focal distance of the lens and appears erect.
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Microscopes. |
Fig. 3134,
a is a jointed microscope for flowers, insects, etc., and is made to fold up and be carried in the pocket.
The holder is movable in the slide for focal adjustment and has forceps between which the object is held.
Simple microscopes, to fold in cases,
b c d e, are made with one or several lenses, which have a diameter of from 3/4 inch to 1 3/4 inches. They are variously mounted.
The
Stanhope lens
f was invented by Earl Stanhope, and is a cylinder of glass with two convex ends.
It is 1 1/2 inches in length and 1/4 inch in diameter, and is mounted in a metallic tube.
The ends have a relative convexity of 6 to 1, and the instrument has considerable power.
The “
Craig” microscope comprises a single lens mounted on a stand, a glass for holding objects below it, and a mirror for reflecting light on the under side of the object and lens.
The magnifying power is greater than that of the cheapest compound microscopes, and the instrument simple and convenient to use.
The compound microscope usually has three lenses, — the object-glass, the field-glass, which intercepts the extreme rays passing through the object-glass and thus enlarges the field of view, and the eye-glass.
It shows objects in an inverted position.
See also objective.
The United States army microscope, made by Zentmayer, is thus described: It has a brass body, 16 inches high, on brass stand, with joint to incline it to any angle, double-milled head-rack and pinion for coarse adjustment, micrometer-screw for fine adjustment, and movable glass stage; under the stage a tube is fitted for carrying the accessory illuminating apparatus, concave and plane mirrors, arranged for direct or oblique illumination, two eye-pieces, one achromatic object-glass 8/10 of an inch focus, of 24 degrees angular aperture, one achromatic object-glass 1/5 of an inch focus, of 80 degrees angular aperture (not adjustable for glass cover), giving power of 50, 100, 250, and 450 diameters: camera lucida, stage micrometer ruled 1/100 and 1/1000 of an inch, and a condensing lens two inches diameter on separate stand.
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Compound microscope. |
Fig. 3135 represents a microscope especially designed for dentists' use. It has a rack and pinion for approximate, and a milled headed screw for accurate, adjustment of the focus, and is provided with two objectives having magnifying powers of 125 and 230 diameters respectively.
It has also two mirrors, a plane and a concave, a revolvable and removable diaphragm, and a condensing lens on a separate stand.
Daylight or strong artificial light suffices for the observation of an object under a lens or microscope of moderate power, but with high powers more light is required, especially as the object is shaded by the lens.
The condenser, a convex lens placed at the side of the object which is in its focus, is then employed.
The light may be also concentrated on the object by a reflector.
Lieberkuhn, 1740, devised a hollow concave reflector with a hole in the center, in which was placed a
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small lens, the foci of the lens and mirror nearly coinciding.
This throws abundance of light on an object placed in the focus, but as it comes equally from all sides no shadows are formed and the object is difficult to define.
The device is, however, well adapted for the detection of small particles of metal, etc., in ores.
The
Abbe Huc exhibited his microscope to the Regent of Thibet at Lha-Ssa.
“An exceedingly robust specimen of the
pediculus was kindly furnished by a noble
Lama, the secretary to the Regent, from his own preserves.
When seized by the nippers, the Lama objected that the experiment would cause the death of a living creature.
‘Be silent,’ said the Regent.
‘
Tsong-Kaba!’
cried he, when invited to apply his eye to the glass; ‘the creature is as big as a rat.’
After looking at it a few moments, he hid his face with his hands, saying it was too horrible to look at. Every one approached the microscope in his turn, and every one started from it with cries of horror.
The
Lama plead for his little parasite.
We removed the pincers and let it fall into the hand of its proprietor.
Alas! the poor victim was motionless.
The
Regent said to his secretary, laughing, ‘I think your animal is indisposed.
Give it some medicine.’
” —
Abbe Huc's Travels in Tartary and Thibet, 1844 – 46.
The solar microscope consists essentially of two lenses and a mirror.
The mirror reflects the sun's rays upon the first lens, by which they are concentrated on an object placed in its focus, and is still farther magnified by a second lens, which throws the image on a screen at some distance behind it. The exterior lens is placed in a hole through the shutter of a darkened room, and the mirror is adjustable so that it may receive the sun's rays under a proper angle to be reflected to the lenses.
The object is here supposed to be transparent; if it is opaque, it must be so illuminated that the rays reflected by it shall pass directly to the interior lens.
The theory of the solar microscope is said to have been invented by
Dr. Hooke, and was described by
Professor Balthasaurs in Erlange in 1710; but its practical execution is due to
Dr. Lieberkuhn in
Berlin, who made the first successful apparatus in 1740.
It is founded on the same principle as the
camera obscura, only in place of causing the lens or lenses to form inside of the apparatus a small image of large exterior objects, as is the case with the
camera obscura, on the contrary, in the solar microscope, the lens or lenses form outside of the apparatus a large image of small objects placed inside the apparatus.
When the objects are opaque and illuminated in front by reflecting mirrors, the apparatus has been called a
megascope.
The apparatus of the hydro-oxygen and the electric microscope is identical with that of the solar microscope, only that for the solar rays, which are not always at command, the so-called
Drummond, lime, hydro-oxygen, or electric light is substituted.
The magnesium light has also been used in this way. Either of the above is inferior to the solar microscope in the same ratio that our best artificial lights are inferior to the solar light.
A modification of the above has been adapted for producing enlarged photographs of small objects.
The enlarged photograph on glass is then placed in the magic-lantern or stereopticon, and re-enlarged for the benefit of the audience.
Or the enlarged view may be taken upon the prepared surface of a wooden block, ready for the skill of the engraver.
A beautiful set of engravings, thus produced, were given in
Harpers' Magazine some years back.
The mode of obtaining enlarged prints from negatives of ordinary size is explained under solar camera. We are much indebted to
Mr. Shive of
Philadelphia for the improvements in this apparatus.
The variety of objects which afford interesting subjects for examination by the microscope is infinite.
For such, the reader is referred to works treating the subject specially.
It is also useful for detecting adulterations in food, drugs, and fabrics, and in many branches of scientific and medical investigation its aid is indispensable.
Another use to which it may be applied was disclosed during the late Franco-Prussian war. Copies of newspapers, reduced many-fold in size by photography, were fastened in large numbers to carrier-pigeons and introduced into besieged cities.
These were easily read by the microscope.
Microscopic writing has of late years attracted some attention.
It is done by means of an instrument composed of a series of connected levers.
The operator writes or traces with the longest lever of the system, while the shortest, which carries a marking instrument, faithfully copies whatever may be written.
It was calculated that with
Peters's machine, at the
London Exhibition of 1862, the whole Bible might be copied 22 times in the space of one square inch.
It has been proposed to apply this invention to practical use by making private marks, visible only under the microscope, to bills and other papers of value, as a protection against fraudulent imitations; being so minute, a counterfeiter would fail to perceive them, while they would be easily recognized by the proper parties.
See Micrograph; Micropantograph.
The prices, as well as the construction, of microscopes vary greatly; those having a magnifying power of less than 200 may be made comparatively simple and at a relatively cheap rate, but when the magnifying power exceeds 300 the whole apparatus becomes more complex, and must be more carefully made in order to handle, focus, and illuminate the objects properly, center the lens correctly, etc. In these the stand alone costs nearly $100, the eye-glasses $5 to $10 each, and the object-glass on Selligne's system from $10 to $75 each, according to power.
Besides eye-pieces and objectives of different powers, there are a number of necessary or useful appendages, as, condensing lens, prismatic illuminator, dark well, parabolic reflector, lieberkuhn, polarizing prism, micrometer, camera for delineating objects, etc., which tend to augment the price, so that a complete compound microscope costs from $300 to $1,000. The binocular microscope is, of course, still more expensive, the price in some instances reaching $15,000. A good instrument, however, amply sufficing for the wants of ordinary amateur observers, may be obtained for $100.
See “Carpenter on the microscope” ;
Hogg's “History of the microscope,” etc.
The investigations of Leuwenhoeck, Schwammerdam, and the earlier microscopic inquirers, were conducted by means of single lenses, the compound microscope being esteemed comparatively untrustworthy, its true theory being imperfectly understood.
In 1829,
Mr. J. J. Lister published the results of his investigations into the laws governing the aberrations of lenses, and gave practical directions for correcting them by combinations of lenses so arranged that the aberrations should balance each; from this period date the great and rapid improvements which have since been made in the compound microscope.
Among recent important improvements in the microscope is the illuminator of
Professor Hamilton L. Smith, for opaque objects, in which the light is
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received upon a mirror behind the object-glass, through which it is reflected upon the object.
Also his mechanical finger, by which objects invisible to the naked eye are picked up on the point of a hair, separated according to kind, and arranged conveniently for observation.
After the discovery of the theory of binocular vision by
Wheatstone in 1838, and especially after the construction of the lenticular stereoscope by
Sir David Brewster in 1852, the idea naturally occurred of applying this principle to microscopes.
This was first effected by
Professor J. L. Riddell of the
University of
Louisiana.
He employed a pair of small rectangular prisms immediately behind the objectglass; these received the incident rays and reflected them through a second pair of similar prisms to the eye of the observer.
This arrangement had the disadvantage of reversing the stereoscopic effect, projections appearing as depressions, and
vice versa.
The first arrangement for producing a correct stereoscopic effect was invented by
Mr. Nachet, and consisted of an equilateral prism which divided the pencil of rays into two equal parts, reflecting them to two similar prisms, right and left hand, which reflected them to the eye of the observer.
The
Wenham binocular (
Fig. 687, p. 285), in which the compound pencil is divided by means of a trapezoidal prism, has, however, on account of its greater simplicity of construction, been more generally employed.
Mr. Nachet has since introduced a simpler form of binocular microscope, which may be applied to an ordinary instrument without altering its construction or impairing its usefulness as a monocular.
This is effected by making an opening in the side of the tube, into which is introduced a rectangular prism which reflects half the pencil of rays horizontally; this is received on another prism, which directs it to the eye of the observer.
Arrangements are made for varying the distance between the tubes to accommodate them to the varying distances between the eyes of different persons.
The disadvantage common to binocular microscopes on this plan is that they perform well only with comparatively low magnifying powers.
This led to the suggestion a few years ago by
Mr. Wenham of a method of dividing the pencil by means of a transparent plane reflector, which, adjusted at a proper angle, might reflect one half the light of each pencil, allowing the other half to pass through.
This plan involves the loss of the stereoscopic effect, but secures to the observer the satisfaction of using both eyes in viewing an object.
It has, however, proved somewhat difficult to carry out this idea satisfactorily in practice.
See prism.
Messrs. Powell and
Lealand of
London have patented an arrangement, consisting of a quadrangular prism which receives the rays, part of which pass through, while the remainder are reflected to a triangular prism which transmits them to the eye. This has the disadvantage of causing the object to appear unequally illuminated in the two tubes.
Mr. R. B. Tolles has constructed an instrument designed to remedy the defects of the original binocular microscope, at the same time permitting the ordinary single-tubed instrument to be used as a binocular.
The double eye-piece has prisms similar to those employed in
Nachet's earliest form, placed at a distance from the objective about equal to that of ordinary eye-pieces.
In this form it is necessary that the axes of the pencils of rays should cross each other a second time, and for this purpose an erecting eyepiece is employed.
Professor H. L. Smith has proposed a form of binocular, consisting of a thin plane reflector, inclined at an angle of 80° to the axis of the telescope; a ray of light from the object, striking the surface of the mirror, is in part transmitted without change of direction, and in part reflected to a triangular prism having one of its angles truncated, where it undergoes a second reflection to correct the reversal produced by the first, and is transmitted along the axis of the secondary tube.
By this arrangement, the intensities of the light of the directly viewed and reflected images appear sensibly equal.
It is obvious that, after the division of the beam of light from the object, it is merely a matter of detail to give each of the divisions a direction proper to be viewed by two or more observers.
This affords great advantages, not only in anatomical and other investigations, but, where the instrument is used merely for curiosity, it enables several persons to observe at the same time, thus greatly economizing time.
In the triple microscope for three observers, the beam of light is divided into three equal parts by a combination of suitable prisms, each part being directed into its proper tube.
Provision is made for the general adjustment of the lenses, and also for the adjustment of the eye-piece of each tube to the eye of the particular observer.
The construction of the
stand is a matter of considerable importance, as upon its arrangement largely depends the degree of illumination and the distinctness with which the objects can be viewed.
A good arrangement of this kind, constructed by
Tolles, comprises a movable stage having two tables, one facing upward and the other downward, firmly connected together and controlled by milled heads.
When the object is placed on the lower table, it is secured by spring clips.
In this case, the central part of the upper table is removed, and the objective descends through the opening.
Any degree of obliquity of illumination, even up to 90°, is thus available for viewing the object.
The stage is also capable of rotation on its optical center throughout the entire circle.
