248 MICRO-SPECTROSCOPE CCh. VIII 



The Amid prism is in a special tube which is hinged to the ocular 

 and held in position by a spring. It may be swung free of the ocular. 

 In connection with the ocular is the sUt mechanism and a prism for 

 reflecting horizontal rays vertically for the purpose of obtaining a 

 comparison spectrum (§ 404). Finally, near the top is a lateral tube 

 with mirror for the purpose of projecting an Angstrom scale of wave 

 lengths upon the spectrum (§ 405, fig. 145, 148). 



In accordance with the above statements the dispersion or separa- 

 tion into colors is given by the flint-glass prism or prisms and follow- 

 ing the general law that the waves of shortest length, blue, etc., will be 

 bent most, the colors have the position indicated in fig. 139-142, 146, 

 149. But if one looks into the direct vision spectroscope or holds the 

 eye close to the single prism (fig. 145), the colors will appear reversed 

 as if the red were more bent. The explanation of this is shown in fig. 

 14s, 2, where it can be readily seen that if the eye is placed at E, 

 close to the prism, the different colored rays appear in the direction 

 from which they reach the eye and consequently are crossed in being 

 projected into the field of vision and the real position is inverted. 

 The same is true in looking into the micro-spectroscope. The actual 

 position of the different colors may be determined by placing some 

 ground-glass or some of the lens-paper near the prism and observing 

 with the eye at the distance of distinct vision. 



§ 392a. The author wishes to acknowledge the aid rendered by Professor 

 E. L. Nichols in giving the explanation offered in § 392. 



Various Kinds of Spectra 



By a spectrum is meant the colored bands appearing when the 

 Ught traverses a dispersing prism or a diffraction grating, or is affected 

 in any way to separate the different wave lengths of Ught into groups. 

 When daylight or some good artificial Ught is thus dispersed one gets 

 the appearance so famiUar in the rainbow. 



§ 393. Continuous spectrum. — In case a good artificial Ught, 

 as the electric light, is used, the various rainbow or spectral colors 

 merge gradually into' one another in passing from end to end of the 

 spectrum. There are no breaks or gaps. 



