again the distance of the vertical scale from the eyehole. Divide each observed magnifying power by the corresponding length and take the mean of the results. This is the value of

dn

fofi

Dividing this by dn, which in the above case was taken to have the standard value 25 cms., we have the value of

1

fofi

If the

may therefore

focal length of the objective fo is known we calculate fi the focal length of the eyepiece. Tabulate observations and results as follows: Microscope B, objective 3.8 cms. (f), eyepiece C.

In normal use the microscope is focussed so that the image seen is at a great distance from the eye in order that the muscles of the eye may be at rest. This reduces slightly the magnification obtained.

SECTION XXXIX.

ADJUSTMENTS AND USE OF THE SPECTROSCOPE.

Apparatus required: Spectroscope, platinum wires for beads, salts, and crayons.

S'

Fig. 72.

E

Vision through a prism. Let a luminous point S (Fig. 72) send out a pencil of homogeneous light, the rays of which, after refraction through the prism, seem to diverge approximately from a point S'. An eye at E will therefore see the luminous point in a displaced position, the amount of displacement depending on the refractive index of the prism for the rays and on its angle. If the light sent out by the luminous point is not homogeneous, but is of two kinds having different refractive indices, there will be two separate images side by side, which will be coloured differently if the difference in the refractive index is sufficiently great. If, finally, the luminous point sends out white light, there will be an infinite number of images shewing a succession of all the colours of the spectrum from red to violet.

Seal about four or five centimetres of platinum wire of 2mm. diameter into a piece of glass tubing, and bend the end of the wire into the shape of a loop thus:

Fig. 73.

Wet the loop slightly, dip it into a mixture of common salt and borax in equal proportions, and heat carefully in the edge of the flame of a Bunsen burner, until the salt fuses and forms a transparent bead. If the loop is not completely filled by the bead, place some more of the substances on it and repeat the process of fusion. Then fix the glass tube to the stand provided, and place it so that the bead touches the flame and colours it. The platinum wire should dip slightly downwards (Fig. 73), otherwise the bead has a tendency to run back along it.

Hold the loose prism provided in front of your eye, and turn your head until you see the displaced image of the flame. Next take a second piece of wire, and having prepared a bead of a lithium salt, place it in the flame along with the sodium bead. Make a sketch in your note-book of what you see. What conclusions do you draw respecting the light sent out by a flame containing both sodium and lithium?

Look now at a luminous flame, again sketch the appearance in your note-book, and explain it.

Cut a slit not broader than a millimetre in a piece of cardboard. Hold the cardboard in your left hand so that the slit is in front of a Bunsen burner containing both a lithium and sodium salt, hold the prism in the right hand in front of your eye and look at the image of the slit. Sketch the appearance in your note-book.

Such a combination of slit and prism forms the simplest kind of spectroscope, and is often useful for the rapid examination of light sent out by a flame, or an electric discharge.

In practice the virtual image of the slit produced by the prism is magnified by being looked at through a telescope. But this cannot be done with advantage without some further

Fig. 74.

change in the optical arrangement, to obviate the so-called "aberration" of the rays. For if rays of light diverging from

a point are traced through a prism, it is found that after emergence they do not accurately diverge from a point, but the section of the pencil will have the appearance of Fig. 74. This does not sensibly affect the sharpness of the image when looked at with the naked eye, or even under small enlargements such as are used in pocket spectroscopes, but when higher power is required the aberration must be obviated. This may be done by the introduction of a lens placed in such a position between the slit and the prism, that the beam of light coming from any point of the slit becomes parallel before it falls on the prism. As a parallel pencil of light will remain parallel after refraction at any number of plane surfaces, there is no aberration in this We thus arrive at the arrangement shewn in Fig. 75.

case.

A tube carries at one end the slit S and at the other the lens L, the slit being in the focal plane of the lens. This tube is called a "collimator." The pencils of light from the slit rendered parallel by L fall on the prism P, and after passing through it are received by a telescope T. The eye, on looking through the telescope will see an image of the slit, which is displaced by the prism, but is sharp provided the slit is illuminated by homogeneous light. If, on the other hand, the light falling on the slit consists of groups of waves differing in wave-length, each group will give a separate image of the slit. If further the wave-lengths, and therefore the refrangibilities, vary continuously over a certain range, the images of the slit will lie side by side or even overlap, so that a continuous band of light will be seen.

We call the appearance presented when the light of a luminous body is examined by means of a spectroscope the "spectrum" of the body. We say that the body has a

"continuous spectrum" when the band of coloured light is continuous. We say, on the other hand, that the body has a “line spectrum," if a number of separate coloured lines are shewn, which we have seen are only images of the slit. If the slit is curved the lines will be curved, if the slit is broad the lines appear broad, and narrowing the slit will narrow the lines down to a certain limit, which depends on the diameter of the object-glass used. A "band-spectrum" is a spectrum consisting of bands which are broad even with a narrow slit. These bands are often sharp on one side and fade away gradually on the other.

As the spectrum of a body, whether it is a line or band spectrum, is found to be characteristic of the body, it is necessary to determine the positions of the lines and bands on some convenient scale.

P

M

T

The arrangement generally adopted in one-prism spectroscopes is shewn in Fig. 76. A small tube MQ has at one end a scale of fine equidistant lines Q, at the other end a lens M, the scale being at the principal focal plane of the lens. The tube is placed so that the light rendered parallel by the lens M, is reflected at the surface of the prism into the telescope T. When the scale is illuminated by a small gas flame, the observer sees not only the spectrum of the body, but superposed on it the image of the divided scale, and he can read off the position of each separate line on this scale.

Fig. 76.

Fig. 77 shews a spectroscope consisting of collimator, prism, telescope, and scale-tube. The collimator is provided with a projecting metal sheet, and the scale-tube with a projecting metal cylinder, to prevent the flames used being brought so near as to injure the instrument.

Adjustments of the Spectroscope: It has been shewn above that the light leaving the collimator should consist of parallel rays. To secure this, if the distance between the slit and collimator lens can be altered, the telescope and collimator are adjusted as follows: