=

4

Suppose, for example, that it is found that one-hundreth of an inch on the stage when measured by the eyepiece required 18 of the eyepiece divisions, then it is obvious that each one of the divisions is of or of an inch; therefore, any object that measured, say four divisions, would be 4 x 1800 1800, or would measure the one four hundred and fiftieth of an inch. There is another method of measurement which is extremely accurate and applicable to all cases; this is, to take a microphotograph of the subject, and to photograph a glass with suitable ruled divisions, with the same arrangements and with the same powers; afterwards a measurement with ordinary compasses can, with great ease and convenience, be made.

Chemical reactions, under the microscope, are either observed in shallow cells ground in the glass slide itself, or simply on the ordinary flat slide, or, as is sometimes convenient, in almost capillary tubes with flattened sides, the microscope being in a horizontal position. Reactions, as a rule, should be observed with only a moderate magnifying power. It is quite possible to execute, on a very small amount of material, a complete qualitative analysis on the stage of the microscope, mixing with drops of the solution under observation droplets of the ordinary test solutions, such as sulphuretted hydrogen water, ammonium sulphide, ammonia, oxalic acid, sodic phosphate, etc. Dr. Beale has recommended glycerine to be used instead of water for these reactions, and he states that although the reactions are slower, yet that they are more perfect.* The method of subliming alkaloids, and its important bearing in the determination of the nature of substances in tea or coffee, is described in the article on "Tea," together with the microscopic appearance of the ash of various leaves, and the method of obtaining "skeleton ashes."

In cutting sections of seeds, leaves, &c., no difficulty is experienced when they are in the entire state, nor are any special instruments required save a sharp razor, for with a little practice sections quite as fine as those it is possible to cut by a sectioncutting machine, can be made with a razor. It is, however, quite different with such matters as tea leaves which have been dried and crumpled, or seeds in the state of powder. Here considerable difficulty may be experienced, and it is often not possible to get a section at all satisfactory of any given dark microscopic particle. The author has had tolerably fair results by sprinkling opaque powders on a piece of smooth wood, and embedding the powders in a tenacious glue. When the cement has set, there is no difficulty in getting sections. Similarly, the known processes for

* "How to Work with the Microscope." London, 1880.

embedding soft substances answer well with tea. A simple method is also to gum the leaf, or fragment of leaf, on to a solid substance, and then horizontal sections can be obtained. Sometimes scraping a leaf in the same manner as when a blot is being erased from paper, brings away very beautiful pieces of the epidermis and stomata. Sections of leaves are easily obtained by placing the leaf between two pieces of cork, pressing them well together, and then cutting the finest possible layers with a sharp razor. In all these cases the razor should be wet with some fluid, either water or (which is for the most part better) glycerine, a little diluted. The section floats on the water, and may be transferred to a dish of dilute glycerine. It is well to cut a great number of sections in this way, and select the most transparent from the dish for microscopic examination. The author's new method of observing and preparing leaves is described in the article on "Tea."

§ 55. Micro-Spectroscope.-The micro-spectroscope bids fair to become a leading instrument in food-analysis,* more especially since the introduction of so many artificial colouring materials. Fig. 8 shows its various parts.

An eyepiece fits into the

The best micro-spectroscope is that known under the name of the "Sorby-Browning" micro-spectroscope.

microscope tube, having the upper lens made achromatic; at the focal point of this lens is fixed a narrow slit. A small rectangular prism is fixed so as to extend over about one-half of the slit, and reflect the light coming through an aperture. In the stage attached to the side of the eyepiece, the other half of the slit transmits the light, passing up the main body of the microscope through the ordinary object glass.

When all is properly arranged and illuminated, in looking through the lens a narrow line of light is seen, one-half the length of which has passed through an object placed on the stage of the microscope, and the other half through any other placed on the side stage attached to the eyepiece of the prism; and if the prism has been properly adjusted, these two portions should appear perfectly continuous, without any break at their junction; but if not properly adjusted the line appears broken, and would then give false results if the spectra were compared together. The analysing prism is compound, and fits over the eyepiece like a long cap. It consists of two rectangular prisms of crown glass, and two others with angles of 75°, a combination which gives direct vision.

B is a milled head adjusting the focus of the eye lens (fig. 8); C is a milled head for adjusting the slit vertically; H for adjusting the breadth of the slit; D,D are springs for holding a small tube; E is for the purpose of regulating the slit of the second spectrum; F is the position of the field lens of the eyepiece; G is a tube which fits on the microscope. The prisms give that amount of dispersion which is admirably fitted for the purposes to which this instrument is applied, and is in itself sufficient to divide the absorption-bands seen in coloured solids and liquids, while it is not so great as to spread them over too wide a space, and make them obscure, as is the case when the dispersion is great. Since the light which passes through the opening does not extend over the same surface as that which passes through the object glass, it would be far too bright unless modified by means of a small shutter, opening and closing with a screw. In each case this can easily be adjusted so that the light from the two sources is equal, or may be made to vary for some special purpose; there is also a contrivance, so that when very small objects are examined no light shall pass except that which has come through them. (Sorby.)

Recent improvements have been made by Mr. Sorby and Mr. Browning, by which every line or band in the spectrum, when being measured, is brought into the centre of the field of view; the jaws of the slit open equally, so that, whatever their width may be, the zero remains unchanged. The micrometer is self

registering, and the whole turns of the micrometer screw, as well as fractional parts, can be read off at the same time by inspection. The instrument may also be used for opaque as well as transparent objects, and two spectra can be compared at the same time with one lamp. Moreover, the spectrum of the smallest object, or a particular part of any object, may be obtained without difficulty. Mr. Sorby's method of measurement is of the most accurate description. He uses an apparatus giving an interference spectrum, divided by black bands, all of equal optical value. The apparatus is composed of two Nicol's prisms, with an intervening plate of quartz, about 043 inch quick thick, cut parallel to the principal axis of the crystal, the thickness being so adjusted with the sodium line that the sodium line is exactly at 3-5, counting the bands from the red end towards the blue. He makes use of the following symbols to express the intensity of absorption :

Not at all shaded,

Very slightly shaded,

Decidedly shaded,

More shaded,

Strongly shaded, but so that a

trace of colour is seen,

Still darker,

Nearly black,

Blank space.

Dots wide apart.

Dots closer.

Dots very close.

Definite narrow absorption-bands are indicated by * printed over their centre. It is assumed that there is a gradual shading off from one tint to the other, unless the contrary is expressed, which is done by means of a small vertical line, as in the following example :

Normal chlorophyll in alcohol (deep green),

Nothing could be more accurate than Mr. Sorby's method of measurement, and for the actual worker his system of notation will also be found most convenient. For the purpose, however, of graphical illustration, "Vogel's" method is preferable, and it has been used in this work to delineate various coloured spectra. The system may be at once understood by reference to the diagram (fig. 12). The amount of absorption is shown by curves. Where the curve is highest, there the band is blackest; where it is lowest or absent, the least absorption is present. There is no doubt that the most permanently useful way to express spectra, whether absorption or spark, would be by wave lengths. Then, • Hermann Vogel: "Praktische Spectral Analyse." Nordlingen, 1877.

however the scales of different spectroscopes might differ (and scarcely two will give the same values), still the results would be the same for all spectroscopes. The following values of wave lengths are sufficient for absorption spectra :

By constructing a diagram similar to the following one,

only many times larger, having the values of the scale marked