USE OF THE MICRO-SPECTROSCOPE 



325 



the frame, D D, adapted to receive it. In either case this second 

 spectrum is seen by the eye of the observer alongside of that pro- 

 duced by the object viewed through the body of the microscope, so 

 that the two can be exactly compared. 



The exact position of the absorption bands is as important as 

 that of the Fraunhofer lines ; and some of the most conspicuous of 

 the latter afford fixed points of reference, provided the same spectro- 

 scope be employed. The amount of dispersion determines whether 

 the Fraunhofer lines and absorption bands are seen nearer or 

 farther apart, their actual positions in the field of view varying 

 according to the dispersion, while their relative positions are in 

 constant proportion. The best contrivance for measuring the 

 spectra of absorption bands is 

 Browning's bright-line micro- 

 meter, shown in fig. 271. At-H 

 is a small mirror by which light 

 from the lamp employed can be 

 reflected through E D to the 

 lens C, which, by means of a 

 perforated stop, forms a bright 

 pointed image on the surface of 

 the upper prism, whence it is 

 reflected to the eye of the ob- 

 server. The rotation of a wheel 

 worked by the milled head, M, 

 carries this bright point over the 

 spectrum, and the exact amount 

 of motion may be read off to 

 loooo^h inch on the graduated 

 circle of the wheel. To use this 

 apparatus, the Fraunhofer lines 

 must be viewed by sending bright 

 daylight through the spectro- 

 scope, and the positions of the 

 principal lines carefully measured, 

 the reading on the micrometer- 

 wheel being noted down. A 

 spectrum map may then be drawn F IG- 271. 

 on cardboard, on a scale of equal 

 parts, and the lines marked on it, as shown in the upper half of 

 fig. 272. The lower half of the same figure shows an absorption 

 spectrum, with its bands at certain distances from the Fraunhofer 

 lines. The cardboard spectrum map, when once drawn, should be 

 kept for reference. 1 



A beginner with the micro-spectroscope should first hold it up to 

 the sky on a clear day, without the intervention of the microscope, 



1 Mr. Swift has devised an improved micro- spectroscope, in which the micro- 

 metric apparatus is combined with the ordinary spectroscopic eye-piece, and two 

 spectra can be brought into the field at once. Other improvements devised by 

 Dr. Sorby and a new form devised by Mr. "F. H. Ward have been carried into 

 execution by Mr. Hilger. (See Journ. of Roy. Microsc. Soc. vol. i. 1878, p. 326, 

 and vol. ii. 1879, p. 81.) 



-Bright-line spectro-micrometer. 



