Part 8

Place the lamp lighted before the glass lump n, and the object you intend to examine between the spring plates of the stage, and the instrument is ready for use.

In all microscopes, there are two circumstances which must be particularly attended to; the modification of the light, or the proper quantity to illuminate the object; secondly, the adjustment of the instrument to the focus of the glasses and the eye of the observer. In the use of the lucernal microscope there is a third circumstance, which is the regulation of the guide of the eye, each of which I shall consider by itself.

1. To throw the light upon the object. The flame of the lamp is to be placed rather below the center of the glass semiglobe n, and as near it as possible; the concave mirror o must be so inclined and turned, as to receive the light from the semiglobe; and reflect it thence upon the object; the best situation of the concave mirror, and the flame of the lamp, depends on a combination of circumstances, which a little practice will best point out.

2. To regulate the guide for the eye, or to place the center of the eye piece L, so that it may coincide with the focal point of the lenses, and the axis of vision. Lengthen and shorten the tubes M N by drawing out or pushing in the inner tube, and raising or depressing the eye-piece M L, till you find the large lens, which is placed at the end A B of the wooden body, filled by an uniform field of light, without any prismatic colours round the edge; for, till this piece be properly fixed, the circle of light will be very small, and only occupy a part of the lens; the eye must be kept at the center of the eye-piece L, during the whole of the operation; which may be rendered somewhat easier to the observer, on the first use of the instrument, if he hold a piece of white paper parallel to the large lenses, removing it from or bringing it nearer to them, till he finds the place where a lucid circle, which he will perceive on the paper, is brightest and most distinct, then to fix the center of the eye-piece to coincide with that spot; after which a very small adjustment will set it perfectly right.

3. To adjust the lenses to their focal distance. This is effected by turning the pinion a, the eye being at the same time at the eye-piece L. I often place the grey glass before the large lenses, while I am regulating the guide for the eye, and adjusting for the focal distance.

If the observer, in the process of his examination of an object, advance rapidly from a shallow to a deep magnifier, he will save himself some labour by pulling out the internal tube at O.

The upper part f g r s of the stage, is to be raised or lowered occasionally, in order to make the center of the object coincide with the center of the lens at O.

To delineate objects, the grey or rough ground glass must be placed before the large lenses; the picture of the object will be formed on this glass, and the outline may be accurately taken, by going over the picture with a pencil.

The opake part may be used in the day-time without a lamp, provided the large lenses at A B be screened from the light.

TO USE THE LUCERNAL MICROSCOPE IN THE EXAMINATION OF TRANSPARENT OBJECTS.

The microscope is to remain as before: the upper part f g r s of the opake stage must be removed, and the stage for transparent objects, represented at Fig. 4, put in its place; the end, Fig. 9 and 10, to be next the lamp.

Place the rough glass in its groove at the end A B, and the objects in the slider-holder at the front of the stage; then transmit as strong a light as you are able on the object, which you will easily do, by raising or lowering the lamp.

The object will be beautifully depicted on the rough glass: it must be regulated to the focus of the magnifier, by turning the pinion a.

The object may be viewed either with or without the guide for the eye; a single observer will see an object to the greatest advantage by using this guide, which is to be adjusted as we have described, page 73. If two or three wish to examine the object at the same time, the guide for the eye must be laid aside.

Take the large lens out of the groove, and receive the image on the rough glass; in this case the guide for the eye is of no use: if the rough glass be taken away, the image of the object may be represented on a paper screen.[29]

[29] A tin cover is sometimes made to go over the glass chimney of the lamp, Fig. 3, with only a small square aperture in front, sufficient to suffer the rays to pass into the microscope: this, by excluding all extraneous rays, adds in many cases most materially to the effect, particularly by day, and when objects are to be represented on the rough glass or screen only. EDIT.

Take out the rough glass, replace the large lenses, and use the guide for the eye; attend to the foregoing directions, and adjust the object to its proper focus. You will then see the object in a blaze of light almost too great for the eye, a circumstance that will be found very useful in the examination of particular objects; the edges of the object in this mode will be somewhat coloured, but as it is only used in this full light for occasional purposes, it has been thought better to leave this small imperfection, than by remedying it, to sacrifice greater advantages; the more so, as this fault is easily corrected, and a new and interesting view of the object is obtained, by turning the instrument out of the direct rays of light, and permitting them to pass through only in an oblique direction, by which the upper surface is in some degree illuminated, and the object is seen partly as opake, partly as transparent. It has been already observed, that the transparent objects might be placed between the slider-holders kl of the stage for opake objects, and then be examined as if opake.

Some transparent objects appear to the greatest advantage when the lens at 9 and 10 is taken away; as, by giving too great a quantity of light, it renders the edges less sharp.

The variety of views which may be taken of every object, by means of the improved lucernal microscope, will be found to be of great use to an accurate observer: it will give him an opportunity of correcting or confirming his discoveries, and investigating those parts in one mode, which are invisible in another.

TO TRANSMIT THE IMAGE OF TRANSPARENT OBJECTS ON A SCREEN, AS BY THE SOLAR MICROSCOPE.

It has been long a microscopical desideratum, to have an instrument by which the image of transparent objects might be shewn on a screen, as by the common solar microscope; and this not only because the sun is so uncertain in this climate, and the use of the solar microscope requires confinement in the finest part of the day, when time seldom hangs heavy on the rational mind, but as it also affords an increase of pleasure, by displaying its wonders to several persons at the same instant, without the least fatigue to the eye.

This purpose is now effectually answered, by affixing the transparent stage, Fig. 4, of the lucernal to a lanthorn containing one of Argand’s lamps. The lamp is placed within the lanthorn, and the end 9, 10 of the transparent stage is screwed into a female screw, which is rivetted in the sliding part of the front of the lanthorn; the magnifying lenses are to be screwed into the hole represented at 12; they are adjusted by turning the milled nut. The quantity of light is to be regulated, by raising and lowering the sliding plate, or the lamp. N. B. This part, with its lanthorn and lamp, may be had separate from the lucernal microscope.[30]

[30] This effect by the lanthorn and lamp is subject to much limitation in the field of view, or circle of light thrown upon the screen. A circle of not more than from 12 to about 15 inches can ever be obtained with any tolerable strength of light, to shew the most transparent sort of objects that can be found, such as the scale of a sole fish, a fly’s wing, &c. The great difference between the light of the sun and a lamp is a natural obstacle to great performances in this way, and renders them far short of the effects of the solar microscope. The exhibition, however, is considerable, and much deserving of the notice of any observer disposed to this sort of apparatus. Probably, subsequent experiments may yet produce more light on this instrument. The best sort of apparatus for this purpose hitherto made, I shall describe in a following section. EDIT.

APPARATUS WHICH USUALLY ACCOMPANIES THE IMPROVED LUCERNAL MICROSCOPE.

The stage, Fig. 1, f g h i, for opake objects, with its glass semiglobe, and concave mirror, which is moveable upon the bar Q R S T, and set readily to any distance by the screw at a. The glasses o and n are also moveable upon the bar for regulating and adjusting the light upon the object.

The stage, Fig. 4, for transparent objects, which fits on the upper part P S of the foregoing stage. When this is to be applied occasionally to a lanthorn for shewing transparent objects on a screen, &c. it is made of a much larger diameter, to admit of the illuminating lenses at 9, 10, and 11, of greater power of condensing the rays from the lamp.

The sliding tube O P, to which the magnifiers are to be affixed; one end of this is to be screwed on the end B of the wooden body; the magnifier in use is to be screwed to the other end on the inner tube. This tube slides inwards or outwards; it is first used to set the magnifier at nearly the right distance from the object, before the exact adjustment for the focus is made, by turning the pinion at a with Hook’s joint and handle b e.

Eight magnifying lenses in brass cells, Fig. 5. Plate III. these are so constructed that any two of them may be combined together, and thus produce a very great variety of magnifying powers. The cells unscrew to admit of the glasses being cleaned.

A fish-pan, such as is represented at Fig. 6, whereon a small fish may be fastened in order to view the circulation of its blood; its tail is to be spread across the oblong hole at the smallest end, and tied fast by means of the attached ribbon. The knob on its back is to be put through a slit hole on the brass piece, No. 5, of Fig. 4. The tail of the fish is to be brought then immediately before the magnifier.

A steel wire, Fig. 7, with a pair of nippers at one end, and a steel point at the other; the wire slides backwards or forwards, in a spring tube which is affixed to a joint at the bottom, on which is a pin to fit the hole in the leg, No. 6, Fig. 4. This is used to confine small objects.

A slider of brass, Fig. 8, containing a flat glass slider and a brass slider, into which are fitted some small concave glasses. It is for confining small living objects, and when used is placed between the two plates, No. 7, Fig. 4.

A pair of forceps, Fig. 9, by which any occasional small object may be conveniently taken up.

Six large ivory sliders, with transparent objects placed between two plates of talc, and confined by brass rings, and six small ditto with ditto. Fig. 10. The larger ones usually contain a set of Custance’s fine vegetable cuttings.

Fourteen wood sliders, containing on each four opake objects, and two spare sliders for occasional objects; all fitted to the cheeks kl of the stage. Fig. 11.

Some capillary tubes, Fig. 12, to receive small fish, and for viewing small animalcula. They are to be placed between the two plates of the stage No. 7, Fig. 4.

A small ivory double box, containing spare plates of talc and brass rings, for replacing any in the small ivory sliders, when necessary.

A single lens mounted in a tortoiseshell case, to examine minute objects previous to their being applied to the sliders.

Opake objects are easily put on the spare sliders by a wetted wafer; and, for good security, gum water may be added.

For the prices of the lucernal, as well as all the other sorts of microscopes, see the list annexed to these Essays.

A DESCRIPTION OF THE SEVERAL IMPROVEMENTS MADE UPON MR. ADAMS’S LUCERNAL MICROSCOPE. BY THE EDITOR.

The lucernal microscope being unquestionably the only instrument for exhibiting all sorts of opake objects under a brilliant and magnified appearance, was, as formerly constructed by the late Mr. G. Adams, attended with some inconveniences and imperfections. Upon a proper inquiry into various improvements, and from some observations made by myself, I can recommend, as a complete instrument, one with the following emendations, being, in my opinion, the best of any hitherto made known.

The lucernal microscope, when placed up for use, as represented in Fig. 1. Plate III. is of some considerable length. When the eye at L is viewing the image of the object upon the glasses, the objects themselves in the sliders placed at kl at the stage, are without the reach of the hand; so that the indispensible change of the parts of an object, or of one object to another, can only be obtained by the observer’s moving himself from the object to the eye-piece, and vice versa. This adjustment, therefore, proves uncertain and troublesome. The application of rack-work motion to the stage has been contrived and applied to the lucernal microscope by Mr. W. Jones, of Holborn, accompanied with Hooke’s joint and handle, and a lever rod; so that, without altering his position, the observer may change both the horizontal and vertical position of the sliders, and thereby readily investigate all the variety of the objects, and their parts, and with the same exactness as by other microscopes.

For persons who may not wish to be at the expense of the lucernal, as formerly mounted by Mr. Adams, Mr. Jones has altered the manner of its support; which, as well as the other particulars, and the method of using it, may be understood from the following description.

Plate IX. Fig. 3, is a representation of the instrument as placed up for use. AA, the top of a mahogany chest, about two feet long, thirteen inches and an half high, and eight inches broad, which serves both as a case to contain the instrument, and to support and preserve it steady when in use. A groove is cut in the top of the box, and another in the inside at the bottom, in both of which the base of the instrument is made to slide. When the instrument is placed inside, a long slip of mahogany slides in at the top, to secure the groove, and make the top perfect. Thus the instrument may be most readily slid out of its case, and then into the groove at top for use, and in much less time than by the brass frame and jointed stand adopted by Mr. Adams. Fig. 3 B, is the stage for the objects, with the condensing lens _a_, and concave mirror _b_, the same as in Mr. Adams’s. C, the brass slider case for opake objects, with a rack cut into its lower edge, and which is turned by a pinion. To this pinion is applied an handle, D, with Hooke’s universal joint; this contrivance gives a certain horizontal motion to the objects while viewing. The stage at C is also made to slide vertically, and a lever-rod or handle, E, to apply through the top, to bring the objects to a just height. Hence, by applying the left hand to the handle, E, and the right to the rod D, the adjustment or the changing of the objects, while under exhibition on the large lenses at F, is produced in the most convenient and accurate manner, and the observer has no occasion, for one slider, to shift from his seat or position.

Rack-work might be applied to the vertical motion, but it is not essentially necessary; for when once the center of the slider is observed, there requires very little change from that position for the complete exhibition of the objects. The whole of the stage, with the lense and mirror, is fixed to a brass dove-tailed slider at G, which slides in another brass piece fixed to the wooden slider or base of the instrument. A long brass rod, H, with an adjusting screw at its end, passes through the two brass pillars, K, K, to the stage at _f_, upon which it acts; and according as it is turned to the right or left hand while examining the objects, moves the objects nearer to or farther from the magnifiers screwed on at L, and produces the just distance for rendering the appearance of the objects the most distinct and brilliant upon the glasses at F.

The management of the light from the lamp, through the lens, _a_, and from the concave mirror, _b_, to the objects, is exactly the same as before directed by Mr. Adams. For the exhibiting of transparent objects, the stage, C, is to be slid away, and the body, Fig. 4, applied in its place, in that position, with the large lens outwards next the lamp. The slider with the objects passes through at _a_, and the focus for the different magnifiers is adjusted by turning the long rod, D, to the right or left, as with the opake objects. In this case the lamp is to be raised to the center of the body of the microscope, or even with the magnifiers at L. The image of the objects may also, as in Mr. Adams’s, be best received on the rough glass placed at F, for the simple reflected light through the body will sometimes be so strong, as to irritate the eye; the operator must, therefore, both modify that from the lamp, and place the roughed glass to his own ease and pleasure. The guide for the eye, N, in this instance is not necessary. Care being taken that the roughed glass at F be kept in as dark a situation as possible, there will be a certainty of a clear and well-defined view of the object.

A tin chimney placed over the glass of the lamp about ten inches long, with a suitable aperture to admit the light to pass through it to the glasses, is of material service; it excludes all superfluous light from the eye of the observer, keeps the room sufficiently darkened, and enables the observer to view his object with the proper brilliancy. As a pleasing relief to the eye, the interposition of a small piece of blue or green glass at the sight hole, N, Mr. Jones has sometimes found necessary, but it gives rather a false teint to the colour of the objects.

In the year 1789 the same artist applied a brass screw pillar and arm to the top of the box at O, on which is occasionally slid the condensing lens, _a_. The lamp being then applied at the side of the box at O, instead of the end, and the lens, _a_, moved to such a distance as to give the strongest possible light upon the opake objects at C; they were found to be more strongly illuminated by this simple refracted light than by the refracted and reflected light before used. Light is always somewhat diminished by reflection, although condensed; therefore, as it is sometimes best to view the objects from oblique reflected light, and sometimes from direct refracted, he constructs the apparatus so as to give the operator the means of easily using either. The dotted lines, O P, shew the manner that the glass semiglobe, _a_, is occasionally applied to refract or converge directly the light from the lamp to the objects on the stage.

It is scarcely necessary to observe to the reader, the propriety of all the glasses of the apparatus being perfectly clean before the observations; for if, after being laid aside some time, or by dust, &c. they should appear soiled, it will be necessary to wipe them previously with a piece of soft shammy leather usually sent in the box for that purpose, or a clean soft cloth. The two large lenses at F, Fig. 3, may be readily separated by turning aside the two brass screws that act upon a brass ring.

From the various ingenious admirers of this sort of instrument, many improvements and alterations have been suggested; among several that have been communicated, those by the two following gentlemen appear to me the most deserving of notice, and which I shall leave to the reader’s judgment and experience.

The Rev. John Prince, LL. D. now of Salem, Massachuset’s States, North America, a valuable correspondent and friend of our late author, transmitted to him an alteration in the construction; and of which I here insert the brief account, in nearly the words given by Mr. Adams.

Dr. Prince applies a strong joint similar to that of a telescope at about the middle of the center part of the pyramidical box, and a sort of adjusting screw at the large end. The joint is nearly in the center of gravity, so that a very small motion is sufficient to bring any object less than an inch in diameter into the field of view. This motion is effected by two screws at right angles to each other; one screw raising or levelling the body, the other moving it sidewise, the screw at the same time forming a double joint to accommodate the parts to the movement.[31]

[31] A figure of this, with an explanation, as recommended by Mr. John Hill, Wells, in Norfolk, may be seen in the Gentleman’s Magazine, Vol. LXVI. 2d part, page 897. In this particular, as well as in the deviation from the parallel position of the glasses to the surfaces of the objects, I think the construction not so simple and perfect, as that by rack-work and pinion applied by Mr. Jones. Probably, Dr. Prince had not, at the time of his contriving the joint-work to the box, seen or heard of the other method. His subsequent contrivances shew real ingenuity; and to the inquisitive in this instrument, will afford much useful entertainment and advantage.

To secure the image formed upon the rough glass more completely from the light, at times essentially necessary, there is a pyramidical mahogany box, of the same size, to pack, when not used, in the body of the microscope; when in use, the broad end of this screen box is to be slid into the groove, from which the exterior cover at the end has been taken. This method is peculiarly useful in the day-time; as, by screening the large lenses from the light, it may even then be used with satisfaction.

One of the large lenses may occasionally be placed on the outer edge of the screen box, the other lens being taken out; the view on the rough glass is by these means magnified, and appears to greater advantage. But, besides the grey glass used in the former construction, there is a second in this, placed farther within the body, about half way; and, when the large lens is in the screen box, objects appear better in this than in the former way. It has a still greater effect upon those who are unacquainted with the nature of lenses, as it makes them judge the distance and magnitude much greater than they really are, and is therefore more pleasing than the grey glass in front. Only one grey glass can be used at a time; both being removed when opake objects are viewed.

The stage, F, Fig. 5, is considerably different from that at C, Fig. 3; it is judged more convenient and commodious than the other, and serves, with a small alteration, for both transparent and opake objects. A truncated cone can also be here applied for cutting off superfluous rays of light occasionally.

The method of illuminating the objects is also different. The mode now adopted answers better for opake and transparent objects, throws a stronger light, and is more convenient in application. It consists of two lenses, 1 and 2, Fig. 5; the larger one is to be placed at the end of the bar next the lamp. The smaller one to be adjusted so as to give a strong light. A third is also added, to be used occasionally with opake objects; it is to be applied close to the large lens. Experience will shew when it is to be used, or not. By moving the bar, G, on which these lenses are placed round the stage pillar, M, you bring it so much fronting the stage as effectually to enlighten opake objects by means of the lamp. The light thus afforded is received directly, and none lost by reflection. The objects are fixed on circular wheels of wood, see Fig. 7, the brass centers of which, are fitted to the hole, _b_, of the stage, Fig. 5; and about this center they are to be turned by the hand for the changing of the objects.