contained in a wedge-shaped vessel. The right hand corresponds to the red end of the spectrum, and the letters refer to the position of Fraunhofer's lines. The absorptive action of the solution is most powerful at the upper part

CHLOROPHYLL.

CHLORIDE OF URANIUM.

FIG. 41.

of each drawing, which represents the spectrum seen where the layer of solution was thickest, and diminishing towards the lower part of the figure.

There are a variety of other substances which have

DARK BAND IN MAGENTA.

DARK BANDS IN BLOOD.

FIG. 42.

this selective power: thus here is the absorption spectrum of chlorophyll, the green colouring matter of leaves, and here that of chloride of uranium.

If I take a solution of blood, and place the cell

containing it before the slit, we get these distinct absorption dark bands, due to the presence of the blood (Fig. 42). This is the red blood: deoxidized blood gives a different appearance. Here you see the two bands due to the red blood, whilst this portion of deoxidized blood gives only one black band, somewhat similar, but not identical in position with the dark band in magenta which I now throw upon the screen. This subject has been examined by Professor Stokes, who published a

paper on the subject in the Proceedings of the Royal Society in 1864. From this we learn that "the colouring matter of blood, like that of indigo, is capable of existing in two states of oxidation, distinguishable by a difference of colour and a fundamental difference in the action on the spectrum." These two forms may be made to pass one into the other by suitable oxidizing and reducing agents, and they have been termed red and purple cruorine

I have here a drawing of Mr. Stokes's diagram of the blood bands. At the top (Fig. 43, No. 1) you have the position of the two bands of the scarlet cruorine. The deoxidized blood is seen in No. 2 to have only one dark band. By the action of an acid on blood, the cruorine is converted into hæmatin yielding a different absorption spectrum; and this hæmatin is capable of reduction and oxidation like cruorine. The absorption bands of hæmatin are represented in Nos. 3 and 4.

f

e

FIG. 44.

One very interesting point to which I must refer is the fact that the blood, when it contains very small quantities of carbonic oxide gas in solution, exhibits a very peculiar set of bands. And the poisoning by carbonic oxide-for, as is well known, the poison of burning charcoal is due to this carbonic oxide-can be readily detected by the peculiar bands which the blood containing carbonic oxide in solution exhibits; and hence we have these absorption lines coming out as a most valuable aid in toxological research.

I would only in conclusion refer you to the instrument by which all these beautiful absorption phenomena can be observed with delicacy and accuracy. It is simply

1

Such spectro-
This (Fig.

a spectroscope placed in connexion with a microscope (Fig. 44). Here we have the instrument. The eyepiece contains prisms, so placed as to enable the refracted ray to pass in a straight line to the eye. scopes are termed direct-vision instruments. 45) is a diagram showing the structure of the eyepiece which I hold in my hand. This is the first lens of the eyepiece here is the slit, for we must have a line of

FIG. 45.

light in order to get a pure spectrum. Then the light passes through the second lens, the rays are rendered parallel, and then they pass through this triple prism ; and inasmuch as the prisms are placed in this position, we see the spectrum by looking straight at the source of light, or have a direct-vision spectroscope. In this way, then, the absorption bands can be very beautifully seen; and, what is still more important, we can, by means of this little moveable mirror, send through the prism any kind of light, and pass the particular ray which

1 W. Huggins, "On the Prismatic Examination of Microscopic Objects" (Trans. Microscopical Society, May 10, 1865).

we wish to examine along with the other light which comes from the object under the microscope, and so observe the two spectra, one above the other; and thus it is that we can detect, for instance, the presence of blood. Supposing we wish to know whether a substance is blood which we have in solution: nothing is easier than to place a small quantity of the liquid supposed to be blood on the table of the microscope, and to bring a small quantity of blood in a tube, so as to compare the spectrum obtained from the body under examination with that of the body which we know is really blood. This instrument, which in the hands of Mr. Sorby has taught us how to detect 1000 part of a grain of the red colouring matter in a bloodstain, is a most beautiful one, and the method of microscopic spectrum analysis must every year become a more and more trusted and valuable means of research in medico-legal investigations.

In the next lecture I hope to bring before you the simple facts upon which Professor Kirchhoff founded his discovery of the chemical composition of the solar atmosphere.