objects are formed upon the retina.

For further explanation

of the way in which the nerves are affected and the sensation of vision produced, the reader is referred to books Physiology.

on

87. Instruments for Magnifying.-In Art. 79 we saw how a magnified virtual image of an object could be produced by means of a convex lens placed at a distance from the object less than its focal length.

When greater magnifying powers are required the following method is used. Let AB (Fig. 74) be the object and O a convex lens which produces a real image of it at ab. This image can not only be thrown on a screen or viewed directly

An

by an eye placed behind it, but it can also be observed by means of a second convex lens. For this purpose the lens must be placed at a distance from ab less than its focal length. observer looking through the second lens sees at A'B' a magnified virtual image of ab. Instead of examining the object itself with a magnifying-glass he applies the magnifier to an image of the object.

This principle is employed in the construction of compound microscopes and astronomical (refracting) telescopes. The image seen is evidently inverted, but in the instruments named this causes no great inconvenience.

EXPT. 38.-Select two convex lenses for carrying out the method above described. Use as your object a pair of cross-wires, or a piece of groundglass on which a small arrow has been drawn with a fine hard pencil. Illuminate the object brightly by a gas-flame.

The lenses are most easily adjusted as follows. Focus on a second ground-glass screen the real image produced by the first lens. Mark its

Focus the

position with a pencil on the glass.
second lens on this mark by moving it backwards
or forwards until a magnified virtual image of the
mark is seen on looking through the lens. Now
remove the glass and you will see a magnified and
inverted image of the object.

88. Astronomical Telescope.-This consists essentially of two lenses: (1) a large lens of considerable focal length called the object-glass, and (2) a smaller lens (or system of lenses) of short focal length called the eye-piece. When the telescope is directed towards a star, a real, diminished, and inverted image of the star is formed at the focus of the object-glass. This image is observed through the eye-piece, just as if it were an actual object, and thus a second and magnified image is obtained.

89. The Compound Microscope differs

from the telescope in having a small object-glass of short focal length. By placing this at a suitable distance from the object a magnified real image is obtained. The path of the rays within the microscope is shown in Fig. 75. The object-glass O produces at a11 a real image of the object ab. This image, already magnified, is viewed through the eye-piece O' and thus the observer sees at AB a virtual and greatly magnified image.

90. Galileo's Telescope-Opera-Glass.—In the earliest form of telescope, invented by Galileo, the object-glass consists as usual of a convex lens, but the eye-piece is a concave lens. The way in which the lenses act will be understood from Fig. 76. The object-glass C alone would form at ab a real, inverted image of the object AB. But when the concave lens c is interposed, each convergent pencil is converted into a divergent pencil. Thus, the rays which would otherwise converge to the point a are made, by passing through the concave lens, to diverge from a virtual focus at A' (A' being on the secondary axis ac). observer sees at A'B' a virtual image of the distant object AB.

Thus the

An ordinary opera-glass is simply a double-barrelled Galilean telescope. This form of telescope has the advantage of producing an erect image.

EXPT. 39.-Start as in Expt. 38; but instead of using a second convex lens take a concave one. Interpose it between the first (convex) lens and Look through the concave lens and adjust its position until you see distinctly a virtual, erect image of the arrow.

the image on the screen.

CHAPTER IX

DISPERSION AND COLOUR

91. In experimenting with prisms, and in examining the images produced by lenses, you have doubtless already noticed the appearance of certain coloured edges. These are seen even when the object itself is not coloured: indeed they are most distinct when the object is white or when it consists of a hole or slit illuminated by white light. White light in fact is not simple but compound; and by refraction it is split up into its component parts. The composite nature of white light was discovered by Newton.

92. Newton's Experiment.-Newton allowed a beam of sunlight to stream through a small round hole in the windowshutter of a darkened room. In the path of the beam he placed a glass prism so as to refract the light upwards towards the opposite wall. But instead of seeing on the wall a round or oval image of the sun he found that by passing through the prism the light was drawn out into a coloured band of considerable length, violet at the top and red at the bottom (Fig. 77). This he

called the spectrum.

The colours of the

spectrum follow the

Fig. 77.

same order as the tints of the rainbow; they pass imper

ceptibly from red at the one end through all the gradations of orange, yellow, green, and blue, to violet at the other end.

93. We learn from this experiment :—

(1) That white light is not simple but compound: it is the result of a mixture of many colours.

(2) That these colours can be separated by passing the light through a prism.

(3) That various colours have various degrees of refrangibility; violet being the most refrangible and red the least.

94. Dispersion.—Our second law of refraction (as stated on p. 172) is only correct so long as we keep to light of one colour, say yellow light. On entering a refracting medium a violet ray is bent more than a yellow ray, and therefore the index of refraction for violet light is somewhat greater than the index for yellow light; for red light it is less than for either violet or yellow. The difference between the amounts of bending produced by passing through a prism produces a separation or dispersion of the various coloured rays.

95. Newton also placed a second prism behind the first with its edge in the same direction, so as to cause a further refraction. He found that no new colour was introduced; the second prism simply increased the amount of deviation or lengthened the spectrum.

He then turned the second prism round, placing its edge towards the base of the first prism, so that the refractions due to the two prisms were in opposite directions. He now found that there was thrown upon the wall a white image of the sun (slightly displaced, as it would be by passing through a thick plate of glass). We learn from this that the various spectral colours mixed in the proper proportions can be made to recombine and form white light. Other methods of performing this recomposition are given in Expts. 43 and 44 (p. 222).

96. A Narrow Slit necessary.—In Newton's experiments the light was admitted through a round hole. Now, if you imagine such a hole to be divided up into narrow strips, in a direction parallel to the edge of the prism, you will easily see that each strip of light would produce a spectrum of its own :