I VI-: SCIENCE. 



345 



which the Bessemer process is conducted. And it is bj the 

 employment of the spectroscope that the exact moment of the 

 completion of thu process for making steel or pure iron may be 

 determined by a person skilled in the use of uunt. 



niniiituro spectroscope represented in fig. 6 is a most 

 interesting illustration of the principle of refraction. Light H 

 allowed to pans through the slit A, and the manner in which the 

 slit is opened or shut is most simple. The mechanical con- 

 trivance for doing thin cannot got out of order, being entirely 

 regulated by the turning of a milled head at the front of the 

 spectroscope. The slit 1 with a cap, to prevent dust 



iK'cuiimhvting, as this will cause lines to bo apparent in the 

 direction of the length of the solar spectrum. The dast may 

 be removed by opening the slit as wide as possible, and then 

 carefully wiping the edges with the corner of a soft silk hand- 

 kerchief. After the rays of light have passed through the 

 slit, they are collected by an achromatic lens, B, which renders 

 them quite parallel. Tho parallel rays are now refracted and 

 dispersed by the five prisms at c. The position of each prism 

 being reversed alternately, the refraction takes place in opposite 

 directions, and in this manner the refracted ray is kept within 

 the axis, or central lino of the instrument. 



line*. Stub a spectrum toachen us nothing beyond the fact thai 

 light can be decomposed into seven colours. Aa observer, 

 looking at such a spectrum, could not tell the exact source at 

 the light, or say whether it was evolved by incandescent char- 

 coal, lime, or platinum. Booh a par* band of colour* fa 

 called a spectrum oj thefint order. 



If a spirit lamp, burning pure and good spirit, is used a* 

 the source of heat, and a platinum wire, looped at the end and 

 dipped into a solution of common salt, is now held in tho 

 spirit flame, it changes yellow; and if the little hand-spec- 

 trosoope is directed towards it, a yellow line U distinctly 

 apparent, the position of which U towards the red end of the 

 spectrum. 



When a more intense heat is used, such as the electric ace, 

 the sodium line is double, and is then exactly coincident with 

 the dark, double solar line known as Fraunhofer's D line, and 

 shown in Fig. 5. If nitrate or chloride of strontium U used, 

 and placed like the chloride of sodium on the looped platinum 

 wire in the flame, and observed with the spectroscope, the 

 coloured bands are more numerous. There are eight remark- 

 able lines one blue bard, one orange, and six red. All the 

 metals, and their salts, which can be converted into luminous 



ladfeo TM*. 



A.3 different coloured rays are each refracted with its own 

 ' idex of refraction, a long band of various colour is obtained, 

 .:. uprising red, orange, yellow, green, blue, indigo, and violet. 



A partial front view of the instrument, exhibiting the slit, is 

 shown at Fig. 7, which is a perspective view of this most useful 

 and handy spectroscope. 



To obtain a pure spectrum, it is necessary that tho jaws, or 

 sides, of the slit should bo almost closed, so that they only 

 allow a very narrow line of light to pass between them If tho 

 lino of light bo made too broad, the compound colours will be 

 formed by the overlapping and mixture of tho colours, which 

 are widely separated from each other in the pure solar spectrum ; 

 for instance, mauve will be produced by the admixture of red 

 and blue, which, although they can in this way be made to 

 overlap, are, as already explained, at the opposite ends of a 

 pure spectrum. If this little instrument is directed to any 

 other source of light, such as gaslight, an electrical spark, or a 

 fixed star, a different set of lines become apparent. 



In order to distinguish these lines properly, it is necessary to 

 classify the spectra obtained from the various sources of light. 



Thus, the light obtained from the incandescence of two 

 graphite electrodes by the voltaic battery, and called tho 

 electric light, provided tho graphite is tolerably pure, will ex- 

 hibit a continuous band of colours, perfectly free from all black 



gas, give bright lines instead of dark ones; and the various 

 spectra obtained in this manner are called spectra of the tecond 

 order. The fact that metals and their salts will always give 

 tho same coloured bands invariably in some particular part of 

 the spectrum affords a most delicate means of qualitative 

 analysis, which is now generally employed where the presence 

 of a minute quantity of some metallic salt is suspected. By 

 means of spectrum analysis the three-millionth part of a milli- 

 gramme of soda can be easily detected, of lithium the nine-millionth 

 part, of calcium the ten-thousandth part of a milligramme. The 

 spark from the great induction coil, when passed through the 

 air, is always of a light yellow colour, and when examined with 

 the spectroscope, it gives the yellow line of sodium ; and this is 

 derived from the dust always floating in the air, which is con- 

 stantly supplied with particles of salt from the spray carried by 

 the winds from the ocean. 



There is only one more order to discuss, viz., spectra of the 

 third order, of which the best type is the solar spectrum crossed 

 by black bands. Mr. Muggins says, "Spectra of this order con- 

 sist of the spectra of incandescent, solid, or liquid bodies, in 

 which the continuity of the coloured light is broken by dark 

 lines. These dark space* are not produced by the source of 

 the light They tell us of vapours through which the light has 

 passed on its way, and which have robbed tho lijjht, by abaorp* 



