CHAMBERS'S INFORMATION FOR THE PEOPLE. 



led to enunciate the important law, that all bodies 

 which at a certain temperature can emit light 

 rays of a certain class, can also absorb the same 

 rays at that temperature. The rays which are 

 absorbed must have come from an incandescent 

 solid or liquid at a higher temperature than the 

 vapour. From this law it at once follows, that dark 

 lines in a spectrum are produced by absorption, 

 and that the position of the dark lines indicates 

 what must have been the absorbing vapour. 

 Now, when iron is volatilised and burned in the 

 electric light, its spectrum is found to consist of 

 several hundred bright lines, and Kirchhoff had 

 mapped 460 of these. He also found that for 

 every one of these bright lines, a dark line corre- 

 sponded exactly in the solar spectrum. Hence the 

 conclusion was inevitable, that the sun's light must 

 have passed through vapour of iron, and that 

 this iron must exist either in the sun's atmosphere 

 or in that of the earth. The latter supposition can- 

 not be correct, else the spectra of all the stars 

 should contain the iron absorption bands, which 

 they do not ; therefore we are driven to the conclu- 

 sion that iron exists in the sun's atmosphere. In 

 the same way it is found that many of the chemical 

 elements with which we are familiar on the earth's 

 surface exist also in the sun. In addition to iron, 

 there are found copper, zinc, nickel, magnesium, 

 hydrogen, sodium, cobalt, and several others ; but 

 not gold, silver, lead, mercury, antimony, or potas- 

 sium. The spectroscope has also revealed to us 

 something of the chemistry of the stars and of 

 the far-distant nebulae ; we have convincing evi- 

 dence that the fixed stars are burning suns, some- 

 what like our own, but each surrounded by its 

 own absorbing atmosphere. Thus Aldebaran, the 

 brightest star in the constellation of Taurus, is 

 known to contain sodium, magnesium, hydrogen, 

 iron, and some other terrestrial elements ; while 



Betelgeux, a bright star in the shoulder of Orion,, 

 contains sodium, magnesium, and iron, but not 

 hydrogen. 



THE TELESCOPE. 



By means of a concave mirror, or a lens, the 

 image of any object can be formed ; and the size 

 and position of the image depend on the distance 

 of the object, and the focal length of the mirror 

 or lens. If the object be at a great distance, the 

 image is formed at the focus, and is of less actual 

 size than the object, although it is seen subtending 

 the same angle by an eye at the centre of the 

 mirror or lens. It is thus impossible to have an 

 enlarged image of a distant object by means of a 

 single mirror or lens ; but the image near the eye 

 can be magnified by using a magnifying lens or 

 eye-glass. This is the simple principle of the 

 telescope, or instrument for seeing at a distance. 

 The telescope is a reflecting or a refracting one, 

 according as the image is formed by reflection 

 from a polished surface, or refraction through a 

 lens. Let O and e in the diagram be the centres 

 of the object-glass and eye-glass, which are sup- 

 posed to be mounted in cylindrical tubes, that of 

 the eye-glass sliding within the other, so that the 

 distance of the lenses may be slightly varied. The 

 line joining Qe is called the axis of the telescope. 

 If the axis of the telescope be directed to the 

 centre of a distant object, the parallel rays from 

 the upper part of the object, after refraction by 

 the lens O, are brought to a focus at the point a+ 

 Oa being the focal length of the lens. An inverted 

 image, ab, of the object is thus formed ; and if 

 this image be in the focus of the eye-glass, e, the 

 rays from every point of it, as a, will, after refrac- 

 tion, emerge parallel to ae, and will be fit for 

 vision by the eye at E, which will thus see the 



Fig. 27. 



point a in the direction a' ; so that the angle 

 under which the half of the object is seen is Oea ; 

 whereas, without the telescope, this angle at E, 

 which could not be distinguished from that at O, 

 would be AOC that is, aOe. Consequently, the 

 magnifying power of the telescope is the number 

 of times the angle Oea contains the angle aOe ; 

 that is, without sensible error, the number of times 

 the line Oa contains the line ea, or the number of 

 times the focal length of the object-glass contains 

 that of the eye-glass. Suppose, for instance, the 

 object-glass had a focal length of 5 feet, and the 

 eye-glass of I inch, the magnifying power would 

 be 60; that is, a line would appear 60 times 

 longer, a surface 3600 times greater, and a solid 

 216,000 times greater. If the distance of the 

 object from the telescope be not great enough to 

 allow us to consider the rays coming from a point 

 in it parallel, the image is formed at a distance 

 from O, a little greater than Oa, and the eye-tube 



requires to be pulled out ; and the nearer the 

 object at which we are looking, the more must we 

 lengthen the telescope. Again, different eyes 

 having different powers, the eye-glass which is in 

 focus for one observer may not be so for another; 

 and hence the necessity for having one of the 

 lenses movable. 



To find \hs. field of -view of the telescope, let us 

 draw a straight line from O to d, the extreme 

 edge of the eye-glass, cutting the image in b. 

 Then of all the rays falling on the object-glass, 

 and converging to b, to form this point of the 

 image, the half (namely, those passing through 

 the lower half of the object-glass) will pass above 

 d, and the eye will see the point b by half-pencils 

 of light. Taking this as the boundary of the field 

 of view, the angular diameter of the field is twice 

 the angle dOe ; that is, the angle under which the 

 eye-glass would be seen from the centre of the 

 object-glass. As the telescope just described 



