CHAP. II.] 



THE BLOOD. 



lu the annexed drawing the arrangement of the whole apparatus is 

 shewn. 



FIG. 18. 



1. At A is a tube bearing at its distal end a slit which may be made 

 narrow or wide at will and which is provided with a reflecting prism by 

 means of which the spectrum of light from two sources may be simul- 

 taneously observed ; at its proximal end it is furnished with an achromatic 

 lens. 2. In the centre of the instrument is a flint glass prism which 

 receives the parallel rays which have passed through the slit and collimating 

 lens. 3. At B is a telescope into which penetrate the rays which have 

 been dispersed, by the prism. 4. At is shewn a tube bearing at its 

 distal end a scale photographed upon glass and which is illuminated by a 

 lamp as shewn in the engraving. 



In actually working with such an instrument the observer, having 

 thrown a dark cloth over the prism, commences by adjusting the lights so as 

 to illuminate the slit and the scale, and by adjusting the slit at the end of 

 tube A, and focusing the scale in tube C and the telescope B, he endeavours 

 to get a sharply defined spectrum, and immediately above or below it a well- 

 illuminated image of the scale. 



In working with the spectroscope it is of great importance to be able 

 to fix more or less precisely the locality of any line or band which has been 

 observed, and in order to do so various plans may be followed. One most 

 commonly followed is to examine very carefully the spectrum of sunlight 

 and to determine the position of the principal Fraueuhofer lines in reference 

 to the scale of the instrument. The observations are tabulated, or a 

 map drawn shewing the position of these lines, which are to serve as land- 



