LIGHT. 61 



a draw-tube with adjustable slit, and a collimator lens placed 

 between them to render the rays parallel. The prisms placed 

 opposite to one another are made of crown glass and flint glass, re- 

 spectively. The dispersive power of the latter is nearly double that 

 of crown glass, while the deviating powers of the two glasses do not 

 differ much. The result 



is that a beam of light Fl - 22 - 



entering through the slit 

 undergoes but little de- 

 viation from its original 



COlirse, while there is Direct-vision spectroscope. 



sufficient dispersion be- 

 tween its colors to produce a spectrum available for spectroscopic 

 uses. When the luminous flame of a candle, oil, or gas is examined 

 spectroscopically, it shows a continuous spectrum i. e., this white 

 light has been decomposed into its constituents. 



Bright line spectra. If into a non-luminous flame of a Bunsen 

 burner a platinum wire which has been previously dipped into 

 common salt (sodium chloride) be held, the flame will be colored 

 yellow, and if it be examined by the spectroscope there will be seen 

 a bright-yellow line in the yellow part of the spectrum, w r hile no 

 other colors are visible. In thus examining salts of other metals, 

 such as potassium, lithium, calcium, etc., by holding in the flame by 

 means of platinum wire, it will be seen that each one gives a number 

 of characteristic bands or lines in different parts of the spectrum, the 

 remaining portion being quite dark. So characteristic are these 

 spectra of different elements that they furnish a distinctive means 

 of recognition ; and indeed some elements have been discovered by 

 this method. These spectra can be seen only when the substance is 

 heated to a point where volatilization takes place, because gases alone 

 show the property of giving discontinuous, or bright line, spectra, 

 and no two gases give the same spectrum. 



The reason that luminous flames give a continuous spectrum is 

 that the luminosity is due to light emitted by particles of solid carbon 

 floating in the burning gas. This shows that the spectroscope fur- 

 nishes a reliable means of determining whether light proceeds from 

 a luminous solid or a luminous gas, as all luminous solids and liquids 

 give continuous spectra. 



Absorption spectra. If the spectroscope is so arranged that a 

 continuous spectrum is obtained from a strongly luminous flame, or 



