together with the refraction caused by the Canada balsam as to come out at the side at D, while the extraordinary ray pursues its course and emerges at F, both rays being polarized.

B

Fig. 67.

OPTICAL INSTRUMENTS.

Telescopes are instruments for viewing distant objects. They may be divided into two kindsrefracting telescopes and reflecting telescopes. In refracting telescopes a lens, usually called the object-glass, is employed to form an image. In reflecting telescopes a mirror or speculum is used for the same purpose. In both, the image formed is seen by a lens or a combination of lenses called the eyepiece. The method of arranging these parts, together with the auxiliary pieces, determines the particular kind of telescope.

An astronomical telescope consists essentially of two convex lenses-the object-glass and the eye-glass, or of one concave mirror and the eye-glass. An inverted image of a star is produced by the objectglass, or by the mirror, and magnified by the eye-glass. Fig. 68 represents this arrangement.

The rays of light being parallel-for they come from great distances-pass through lens N and form an

H

M

Fig. 68.

inverted image, bc, between the eye-glass and its principal focus. The rays passing through the lens M are refracted, and the eye sees the image much magnified at b'c'. In a good telescope the magnifying power does not exceed 1,000 or 1,200. The image is inverted, but of course that is of no importance in viewing the heavenly bodies.

A terrestrial telescope differs from an astronomical telescope in having two additional convex lenses, which together constitute what is called an erecting piece, which enables the observer to see images erect instead of inverted.

The Galilean Telescope is so called from its inventor Galileo. It is, in fact, a common opera

E

Fig. 69.

glass, which consists of a convex object-glass and a double concave eye-glass. Fig. 69 represents this kind of telescope, where O is a convex object-glass, in the focus of which an inverted image of the object would be formed were it not for the double concave lens E. The converging rays of light are caused to diverge by falling on this lens, enter the eye much less convergent, and form an erect image of the object.

So far we have assumed that the eye-glass consists of one lens, for simplicity of illustration; but it must be borne in mind that in good instruments both spherical and chromatic aberration require correction, and that in terrestrial telescopes an erecting combination is also necessary. The fact is, there are many arrangements of lenses in eyepieces, according to the results aimed at or the opinions of opticians. Campani's, Ramsden's, and Dolland's are the most noted.

A reflecting Telescope is one in which the image of a distant object is formed by means of a polished metal reflector or speculum, which image is then viewed with an eye-glass. The eyepiece may either be a single lens or a combination of lenses. One of these telescopes was constructed by Newton (Fig. 70). It consists of a concave speculum, A A, placed at one end of the tube, and a small plane mirror, C D, placed

2

m

Fig. 70.

obliquely to the axis of the tube. The image of a distant object formed by the speculum A A is reflected by the mirror C D to a point, m n, on the side of the tube, and is there seen through an eyepiece, 7, which is made of two plano-convex lenses. Large reflecting telescopes are now constructed without the small plane mirror C D. This is accomplished by means

of a small rectangular prism, which takes the place of the speculum. The arrangement of the eyepiece is the same.

The Gregorian and Cassegrainian Telescopes. These are the most prevalent forms of the reflecting telescope. The Gregorian will be understood from Fig. 71. The rays enter parallel, strike the mirror at A,* and form an image at B. This image is reflected from a small mirror, C, to the opening in

B

a

Fig. 71.

Much light is lost by reflection.

A, where it is viewed by the eyepiece D, and so enlarged into the image a b. this instrument in the double The Cassegrainian differs from it in having the large mirror curved so as to throw the image to the side of the tube, where it is viewed with lenses. Very large telescopes of this kind are often arranged so that the astronomer can ascend and use the eyepiece in the upper end of the tube; there is then only one reflection, and therefore little loss of light.

A microscope is a modification of a telescope, and is used for viewing near objects. Microscopes are either simple or compound. The simple microscope consists of a double convex lens of short focal distance. It is usually set in a frame of horn or

*The interior of end A is one large mirror with an aperture in the middle for the eyepiece, and this mirror is spherical.

B

Fig. 72.

metal, and held in the hand. It is, in fact, a simple magnifying glass. Fig. 72 represents the magnifying principle of the microscope. An eye at E would see the cross A B under the visual angle A EB; but when the lens is interposed, it is seen under the visual angle C E D, and therefore appears much enlarged, as shown at C D.*

The compound microscope consists essentially of a double convex lens called the object lens, and a second double convex lens called the eye-glass. Fig. 73 shows a section of this instrument. The object to be observed is placed at a, between two plates of glass, on a support. Over this is a tube, in which are placed the two lenses, the object lens, 4, at the lower end, and the eye-glass, L, at its upper extremity. The object, a, being placed a little beyond the principal focus of the object-glass, this lens produces a real image, bc, which is inverted. The object-glass is so placed that its principal focus is a little beyond the image bc, and between the eye-glass and its principal

* Lines EC and ED should be perfectly straight.