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MOUNTING.

with acidulated spirit, or aqueous alum solution, the sharpness of outline given by the colour is decidedly impaired. After staining, the slides must be brought back to balsam by a reverse process; viz., by passing them through (1) water ; (2) methylated spirits ; (3) absolute alcohol ; (4) benzine; (5) mount in balsam and benzine. Turpentine and oil of cloves should not be used for No. 4 (benzine), because they cause many colours to fade, which benzine will fix. Glycerine is not a good mounting medium, for my experience of it is that all stains fade in it.

All these directions may sound very complicated to the tyro, but the method of mounting is really very easy, because the sections are fixed to the slide. There is, therefore, no danger of their floating away when the cover is let down on them, and no trouble with air-bubbles.

STAINING,

In the ordinary way of mounting sections, they are loose, and must be transferred with a “section difter” of some kind from Auid to Auid, as they are washed and stained. This is very troublesome and risky with any sections, and impossible with insects, because they always break up. The safe way, and at the same time the way which produces the best results and is also the least trouble, is to fasten them to the slide as soon as they are cut, and then there is no risk of breaking them. This is done by brushing the face of the slide with a mixture of four or five parts oil of cloves to one part collodion, spreading it thinly and evenly, and covering a little larger space than the section will occupy. Some people prefer glycerine and white of egg, spread very thinly. I lay my slides on a card, the size of a slide, ruled in the middle into squares to of an inch in diameter. Take up a length of ribbon by placing the blade of a scalpel underneath it, and, guided by the card, put the end section down at the right place on the slide, gently draw away the scalpel, and Aatten down the ribbon on to the slide with the back of the blade. See that every part of every section touches the glass and is held by the cement. When all the sections are arranged, take hold of one end of the slide with a clip, and hold it over a flame for a few moments until the wax is melted. The melted wax and the oil of cloves draw away from the sections and form a ring of drops round them. These should be wiped off, because they take the colour when the slide is stained. Plunge the slide into turpentine. A few seconds suffice to dissolve the wax and oil of cloves, and the sections remain on the slide, held fast by the collodion. If staining be unnecessary, or if the object has been stained whole, the turpentine should be washed off with benzine, and the slide mounted in balsam thinned with benzine. If the objects are to be stained on the slide-and this method, I think, gives the best results-proceed as follows. Remove the slide from the turpentine, and wipe off as much of it as possible ; pour a few drops of absolute alcohol on the sections; and, when this has dissolved out the turpentine, put the slide into methylated spirits. It is best to let it soak in this for a quarter of an hour at any rate, in order to extract all the turpentine. Transfer it to clean water, and soak it in that another ten minutes or so. Now stain, leaving it as long as is necessary in the colour. But, if you wish to remove pigment to show the structure of eyes or other pigmented parts, the slide must be left in eau de javelle until all the pigment is dissolved, and then well washed in water, before being stained. The time needed for staining is generally fifteen minutes, but the slide should be examined frequently to see how it is getting on. The sections must not be overstained if hæmatoxylin be used ; for, although overstain can be washed out

A few words on staining may be useful. As I have already said, specimens may be stained whole, before they are imbedded in wax, or stained on the slide after they are cut. Some like one way, some the other. I find it difficult to stain the object whole to just the right tinge of colour ; I either get it too dark or only partially stained, and it is specially difficult with insects because of their impervious shells. If you make holes in them with needles, you damage the internal anatomy. Therefore, I prefer staining the sections on the slide, in the way that I have described.

All sorts of colours have been recommended. Aniline dyes are in great favour with some people, and very nice double stains of vegetable sections may be made with them.

Borax, carmine, and picro-carmine are much used. These colours are much better than aniline dyes, in that they stain less diffusely and more selectively," as it is called. But I reject them, because with any objective higher than a quarter-inch, they give no sharpness of outline.

I use hæmatoxylin, and hæmatoxylin would be the perfection of stains, if-it is a great pity, that “if !”-it did not labour under the suspicion that it fades in a year or two. That it sades sometimes there is no doubt; but it is doubtsul if the fading be due to the fault of the colour, or to the reagents used in preparing the specimen. It certainly fades if the specimens have been hardened in chromic acid, or any chromate ; and I know that turpentine causes it to fade. I have great hopes, however, that, when the specimens have been hardened in simple spirit, washed finally with benzine, and mounted in balsam and benzine, the colour will keep. Some slides that I mounted six months ago in this way show no sign of turning colour as yet ; but six months, of course, is a test not long enough.

A friend of mine, who is very skilful in mounting, says that he considers the cause of the fading of FORMS OF CLOUD IN RELATION TO THEIR COMPONENT PARTICLES.

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hæmatoxylin to be, not in any reagents used in preparing the specimens, but in the presence of tannin in the ordinary hæmatoxylin stains. His recipe for preparing the stain so as to get rid of the tannin may be found in the “ Quarterly Journal of Microscopical Science,” 1885, at the end of an article on the “ Eyes of Insects.”

Perhaps tannin, chromates, acids, and turpentine all contribute to make it fade. My own recipe is as follows; it gives as good results as any hæmatoxylin I have ever seen ; but if fading be because of the presence of the tannin in the logwood, my slides will fade in time.

Crush ordinary extract of logwood to powder, and dissolve a saltspoonful of it in hot water ; add about

oz. of methylated spirit ; dissolve a teaspoonful of alum in some more hot water, and mix the two solutions ; add more water (if necessary) until the bulk equals three ounces. Allow the fluid to stand for some hours; then filter. Keep it a week before you use it, and then filter it a second time.

I have lately stained a few slides with “Carters'' blue-black ink, diluted with water and spirits. It gives very good results, and colours specimens either a pleasant blue-black, or (if you leave the slide a couple of hours in water or spirit after staining), a gray, like the tone of a photogravure. The definition is not quite so sharp as that given by hæmatoxylin, but I have a retina of a spider excellently well stained by it. As I have only used it for a very short time, I should not venture to recommend it as anything very first-rate, but I think it decidedly worth trying. On the other hand, nothing can beat hæmatoxylin if it will only not fade.

I keep my fluids-turpentine, spirits, and the stains-in three-ounce, wide-mouthed, corked bottles. These are tall enough, and the mouths wide enough to take a slide. And so, by dipping the slides first in the one bottle, then in another, as required, the trouble of mounting is greatly reduced. It is a clean process; there is no waste ; and yet you always have ample quantities of fluids. If you have two bottlefuls of each kind in use, you can have six or eight slides in the course of preparation at once.

In conclusion, a hint or two as to the sort of insects to choose for cutting into sections will not be amiss. Those with hard chitine should be avoided, for, not only do their shells notch the razor terribly, but they also crush into the softer parts while the section is being cut, and so spoil it. Small insects are much more easily cut than large ; long and cross sections of whole insects are very instructive Eyes are very interesting, and spiders are particularly good subjects. And, as comparatively little has been done in “sectionising" insects as yet, there is room for the enthusiastic entomologist to discover new facts, and so do a little original work.

E shall have stormy weather, sir ; those

animal clouds have been about again today.” Such was the remark made to me by a country woman in Kent, and this observation was true enough to nature : it agreed also with my own notes at the time on the appearance of “Anvil cloud," or fracto-cumulus. The “Ram's-head” cloud might not inappropriately be the term used to describe this drifting bank of hail or rain, which often marks the sky with such striking and fantastic outlines. Observe these dark rolling masses at sunset on an autumn afternoon, and see what kind of weather the night brings with it--driving sleet, and sudden gust of wind, with “bursts” of hail rising often to a furious and full-blown nor'wester, such as makes one thankful for a good roof overhead.

The general form and type of the “Hail cloud” is pretty well known to observers of atmospheric phenomena. In most cases it greatly resembles the snow-cumulus, though generally more craggy in outline, harder in its edges, and more attended by stratus at its base. There are, however, several types of snow-cloud. When dristing against a clear sky, the latter presents a more fleecy and softer edging, though in its general form it must be grouped, like that which originates hail-showers, with the class of “ Animal ”

cloud, i.c. condensed Fracto-cumulus. Both of these are again closely akin to the Electric cumulus, or Cone-cloud (see SCIENCE-GOSSIP for July, 1879). This is natural enough when we recollect the frequent connection existing between the hailshower and electrical discharges, and the part that electricity is known to play in the condensation and cohesion of the watery particles.

Let us notice now that a “law” seems to hold good in regard to the origin of the different forms of cloud, and that this law is the real principle by virtue of which it is possible to forecast weather from the observation of clouds. This relation, which we may term a " law of correspondence," between the particles composing a cloud and the general form of the mass, varies its manifestations with the temperature and other physical conditions of the medium in which the vapoury particles are floating : yet the principle involved in the connection between the form of the body and the molecules which compose it is constant.

It is a physical or chemical question to determine the special forms that will be originated by given component particles under given physical conditions, To determine such resultant forms, in the case of previously unknown substances, is a problem

* This indicates a closer affinity with the rain cloud or nimbus.

† This is the term adopted by Professor Poäy, and indicates a mass broken by wind.

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difficult in the highest degree, and I suppose practically insoluble in the generality of instances, except where an approximating form may be guessed at, by witnessing the evolutions that result from analogous substances.

To forecast, for example, the general form which would be assumed by an aggregation of the crystals of some hitherto uncompounded chemical, would be an impossibility, except by examining the effect of known combinations almost similar in composition.

Still more difficult would it be to suggest the form likely to be taken by a tree, from an examination of the seed, though even here analogy might suggest something. But it could suggest nothing as to the method of development with regard to the arrangement of particles, nor as to the reason of their

An Owen may rebuild, from the fragments presented to him, the frame of a being long extinct, whose remains lie imbedded in the bowels of the earth, and we accept his construction because we believe it to be based on the order of Nature shown by the exact observation of analogous forms. But even Owen's efforts to reconstruct would be simply abortive, and the result a falsity, were it not for the permanence and continuity of law in Nature. Without the operation of such law within the component particles, no species either in the animal or vegetable world could possibly possess "continuity" of form.

It is sufficient for our present purpose to direct attention to the relation existing between

mass and “particle.” But it is also necessary to point out that there must always be a great defect in the

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taking the particular form under observation. The value of illustrations from the mixing of liquids of seeds of two different plants may be exceedingly differing density, such as those adduced by the late alike in general structure and composition, yet the Professor Jevons. The movements and elasticity of form of their leaves and stem will present the widest the atmosphere vary too greatly from ordinary possible diversity.

“ liquids" to allow of experiments in mixing being Yet in considering the process of growth, we must of much practical value in regard to known forms of not overlook the variety of elements which may be cloud. And in addition to mechanical movements absorbed from the earth and atmosphere. It is not we have earth magnetism to deal with, which, from a question of the simple evolution of a given material its known connection with auroral manifestations, into an organism from a given embryo, but of the may not unreasonably be thought to influence the drawing into a focus or vortex a great variety of aggregation of vapour. elements, operated on by an almost equal variety

SAMUEL BARBER. of forces. The laws which regulate the operation of those forces are the laws which originate the various forms that meet our eyes, and produce a One of the last new things out is a watch whose resulting structure characterised by permanence and face can be lit up at night by a small electric lamp. beauty.

This will prove a useful watch for seafaring men.

CHAPTERS ON COLOUR.

By S. A. NOTCUTT, JUN., B.A., B.Sc.

No. I.

THE natural starting-point in this subject is the

we see by the aid of external light, usually white light. This light, on passing through a prism is, we know, split up into the prismatic colours, forming what is called the spectrum ; and thus we learn that sunlight is composed of rays of light of almost every different colour. On holding a piece of red glass in the path of the rays as they issue from the prism, the green and blue portions of the spectrum disappear, leaving little else than red, for the red glass absorbs the green and blue rays.

Similarly, blue glass would absorb the yellow and other rays, and, in fact, with various glasses and coloured solutions, we can absorb any particular set of rays we like, and in each case the rays that remain after this process of absorption constitute the particular colour of the glass or solution.

This is the case with transparent coloured bodies. In the case of an opaque body, such as a pigment, a portion of the light falling on it is reflected at the surface, and as a rule is not changed in colour ; but another portion penetrates the substance, and since the internal structure of such a substance is always irregular, this portion of the light is soon reflected back; however, in passing and repassing through the superficial layer of the substance, it suffers absorption, and hence issues forth as coloured light, the colour being what we recognise as the colour of the body.

In a leaf a very thin layer of the colouring matter, chlorophyl, is sufficient to absorb all the orange, blue and violet rays contained in the incident light; hence the light reflected back from the interior of the leaf is without these rays, and since the remaining rays together constitute a green light the leaf looks green.

“ Thus,” says Tyndall, “natural bodies have showered upon them, in the white light of the sun, the sum total of all possible colours; and their action is limited to the sisting of that total, the appropriating from it of the colours which really belong to them, and the rejecting of those which do not. therefore say that it is the portion of light which they reject, and not that which belongs to them, that gives bodies their colours."

It is sometimes convenient to be able to compare by means of a diagram the light transmitted by two different media. This is usually accomplished by taking a rectangle to represent the solar spectrum, and in it drawing a curve, the ordinates or distances measured upwards for any point of the curve representing the amount of light which is transmitted of that particular part of the spectrum,

On comparing the diagram of the light reflected by green pigment with that of the green light from vegetation, we find a considerable difference (Figs. 2: and 3). We see that besides a quantity of yellow and green being transmitted by the foliage green, a portion of the extreme red is also transmitted. If we therefore cut off the yellow and green light coming from foliage, we should expect it to appear red, and this is seen to be the case on viewing a garden or field through 'glass stained a deep blue with cobalt. In sunlight a piece of yellow glass. should be added, to cut off the extreme blue and violet.

A sunny landscape viewed through these two glasses presents a curious appearance : green trees and plants are a red colour, the sky is greenish-blue, the clouds purplish-violet, and anything orange appears blood-red. The absorption diagram of these two glasses shows that they cut off almost all the green light furnished by leaves, but transmit the bluish-green. rays which leaves do not furnish.

An interesting phenomenon depending on absorption is that known as dichromatism, which is the variation in the apparent colour of an absorbing medium when different thicknesses are used. Thus. the colour of a solution of litmus enclosed in a wedge-shaped glass vessel, varies from blue at the thinnest part to red at the thickest. Chromium: chloride varies from green to red; potassium permanganate from purple to blue; reduced hæmoglobin from green to purple. Several thicknesses of yellow glass appear red.

This phenomenon depends on the principle that if a certain thickness, say one centimetre, of a medium absorbs a certain proportion of the rays, then the same proportion of the remainder of the rays will be absorbed on passing through another certimetre of the medium. In the case of litmus, suppose that in the incident light there are one hundred blue rays to every ten rays of such wave length, that they are partially transmitted by the solution, whilst rays of any other refrangibility are completely absorbed ;. then after passing through an extremely thin layer, the emergent light will be a deep blue, the proportion. of blue to red being ten to one, and the red being: thereupon scarcely noticeable. Now suppose that each millimetre of the solution absorbs one-tenth of the red and one-half of the blue rays, then after passing through i mm., the light will contain nine red rays to every fifty blue, and after passing through successive millimetres of the solution, the proportion of red to blue rays will be as the table below (taken: from Glazebrook’s “Physical Optics ''), from which it is

apparent that the relative intensity of red to blue in the emergent light has altered from a proportion of i to 10 to one of more than 3 to i as the thickness. has been increased to 6 mms., and the light is finally a reddish-purple, which may be made quite red by: increasing the thickness sufficiently.

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In the case of most natural bodies, we have seen that the colour is due to an absorption of portions of thie light in traversing a small thickness of the substance, and in the subsequent reflection from the in:erior. In these cases the portion of the light rellected from the surface itself is of the same colour as the incident light. With metals and allied bodies, this is not always the case. In the act of reflection at the surface of a metal, a partial selection takes place with regard to the rays. In the light reflected from a sovereign, yellow rays predominate, whilst the interior of a gold-plated vessel shines with a still deeper orange-yellow, owing to the selection exercised by each successive reflection.

Some metals only exhibit colour on such oftenrepeated reflections, Light thus reflected from steel becomes blue; from silver, yellow.

Another remarkable feature about metallic reflection, is the amount of light reflected ; whereas white paper only reflects 40 per cent., polished silver reflects 92 per cent. of the incident light.

It is these peculiarities in the nature of metallic surface which render gilt frame so suitable for enclosing a painting, since it isolates it from surrounding objects, and does not intrude its own .colour upon the painting, since that colour is of a different character to the colour of the pigments used in the painting.

The ordinary colour of metals is then due to the components of white light, which are entirely reflected. If we obtain a sufficiently thin sheet of a nietal, we can examine the light transmitted through it ; and this, as we should expect, will be of a different colour to the ordinary colour of the metal. In the case of gold, the light at the first surface is rubbed of its yellow rays, which are completely reflected, and the transmitted portion consists of blue or bluish-green rays. Gold can be easily precipitated in the metallic state from its solutions, and being then in a very fine state of division, is capable of transmitting light. Such solutions containing precipitated gold, appear bluish-green by transmitted light, and orange-red by reflected light.

There are many other bodies which display similar colour phenomena to metals. In the case of a crystal of permanganate of potash, the light reflected

from the surface is green. The crystal itself is almost opaque, so that it is not easy to observe the colour of light transmitted through it ;, but we find that the colour of a solution of it is a deep purple, well-known as Condy's fluid. Solutions of the aniline dyes when spread on glass and allowed to dry-so as to leave a thin film of colour-display one colour when we look through them, and another when we observe the light reflected from their sursace.

The ordinary aniline ink (used for writing on graphs) is of a violet colour ; but light reflected obliquely from the surface of such writing is applegreen. Light reflected from the surface of a film of blue aniline is bronze.

There is another phenomenon attendant on the reflection of light from metallic bodies, which is that the reflected beam is at no angle completely planepolarised ; i.e. it always consists of two parts, one plane-polarised, and one not; whereas, with most surfaces, at some particular angle the incident beam is completely polarised.

So far we have dealt with questions of colour involving absorption. Let us now examine the results obtained by the mixture of coloured lights which is quite distinct from the mixture of pigments on the painter's palette. Various methods have been adopted for this purpose. Lambert and Helmholtz used a vertical plate of glass, with a piece of coloured paper placed horizontally on each side, and observed the union of the reflected and transmitted colours. When one of the pieces of paper is yellow and the other red, their superimposed image is orange. With blue and yellow papers the image is grey. Maxwell's colourtop is one of the most convenient methods of making colour mixtures. It consists of a spindle capable of making rapid revolutions, on one end of which coloured discs of cardboard, seven or eight inches in diameter, are placed. Each disc has a radial slit to allow of other discs being combined with it, so that a composite disc can be formed with several sectors of different colours. When such a disc is rotated at from twenty-five to fifty revolutions a second, the sensation aroused in the eye by one scctor has not time to disappear before the images of the other sectors are brought to bear, and consequently a complete fusion of the colours takes place. When a yellow and blue disc are combined and rotated, the whole appears a dull grey; vermilion and a bluish-green also yield a grey, as do purple and emerald green. Now a surface is grey which, while it reflects white light, does not reflect so much as white paper or chalk does. Therefore we may say, that these pairs of colours produce by their mixture the sensation of white light of a low intensity. Two colours which do this are said to be complementary. Other colours when combined may produce the sensation of some intermediate colour in the spectrum. For instance, a disc half red and half yellow, when rotated looks orange. A green

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