SPECTRUM ANALYSIS. 147 



be " too coarse-grained for our purposes " ! " Light," says a writer on this 

 subject. " we can then no longer regard as made of smooth rays ; we have 

 to take into account and to our annoyance the fact that its * long levelled 

 rules ' are rippled, and its texture, as it were, loose woven " ! 



Twenty years ago Professors Kirchkoff and Bunsen applied Fraunhofer's 

 method to the examination of coloured flames of various substances, and 

 since then we have been continually investigating the subject ; yet much 

 remains to be learnt of Spectrum Analysis, and Spectroscopy has still much 

 to reveal. From Newton's time to the present our scientists have been 

 slowly but surely examining with the Spectroscope the composition of 

 spectra, and the Spectroscope is noiv the greatest assistant we possess. 



" Spectrum Analysis, then, teaches us the great fact that solids and 

 liquids give out continuous spectra, and vapours and gases give out dis- 

 continuous spectra instead of an unbroken light " (Lockyer). We have 

 found out that the sunlight and moonlight are identical, that the moon 

 gives us spectrum like a reflection of the former, but has no atmosphere, 

 and that comets are but gases or vapours. The most minute particles of 

 a grain of any substance can be detected to the millionth fraction. The 

 iinnr of a grain of blood can be very readily distinguished in a stain after 

 years have passed. The very year of a certain vintage of wine has been 

 told by means of " absorption," or the action of different bodies on light in 

 the spectrum. It is now easy, " by means of the absorption of different 

 vapours and different substances held in solution, to determine not only what 

 the absorbers really are, but also to detect a minute quantity." The appli- 

 cation of this theory is due to Dr. Gladstone, who used hollow prisms filled 

 with certain substances, and so thickened the " absorption lines." By these 

 lines, or bands, with the aid of the Spectrum Microscope, most wonderful 

 discoveries have been made, and will continue to be made. We will close 

 this portion of the subject with a brief description of the Spectroscope in 

 principle. 



The instrument consists of two telescopes arranged with two object- 

 glasses on a stand (fig. 150). A narrow slit is put in place of the eye-piece 

 of one, the arrangement admitting of the slit being made smaller or larger 

 by means of screws. The glass to which the slit is attached is called the 

 collimating lens. The light, at the end of the slit seen from the other 

 telescope, being separated by the prisms between the two telescopes, will 

 produce the spectrum. The Spectroscope is enclosed, so that no exterior 

 light shall interfere with the spectra the student wishes to observe. This 

 merely indicates the principle, not the details, of the Spectroscope, which 

 vary in different instruments. 



We may now pass from the Spectroscope to the Telescope and the 

 Microscope, instruments to which we are most largely indebted for our 

 knowledge of our surroundings in earth, air, and water. 



The word Telescope is derived from the Greek tele, far, and skopein t to 

 see ; and the instrument is based upon the property possessed by a convex lens 



