ACCESSORY APPARATUS. 



91 



for measuring the spectra of absorption bands is Browning's Bright-line 

 Micrometer shown in Fig. 63. At K, 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 observer. The rotation 

 of a wheel worked by the milled-head M 

 carries this bright point over the spec- 

 trum, and the exact amount of motion 

 may be read off to 1-1 0,000th inch on 

 the graduated circle of the wheel. To 

 use this apparatus, the Fraunhofer lines 

 must be viewed by sending bright day- 

 light through the spectroscope, and the 

 positions of the principal lines care- 

 fully measured, the reading on the 

 micrometer-wheel being noted down. 

 A Spectrum-map may then be drawn 

 on cardboard, on a scale of equal parts ; 

 and the lines marked on it, as shown in 

 'the upper half of Fig. 64. The lower 

 half of the same figure shows an Ab- 

 sorption-spectrum, with its bands at 

 certain distances from the Fraunhofer 

 lines. The cardboard Spectrum-map, 

 when once drawn, should be kept for 

 reference. 1 



89. A beginner with the Micro- 

 Spectroscope should first hold it up to 

 the sky on a clear day, without the 

 intervention of the microscope, and note 

 the effects of opening and closing the slit by rotating the screw c (Fig. 61); 

 the lines can only be well seen when the slit is reduced to a narrow open- 

 ing. The screw H diminishes the length of the slit, and causes the 

 spectrum to be seen as abroad or a narrow ribbon. The screw E (or in 

 some patterns two small sliding-knobs) regulates the quantity of light 

 admitted through the square aperture seen between the points of the 

 springs D D. Water tinged with port wine, madder, and blood, are good 

 fluids with which to commence this study of absorption-bands. They 

 may be placed in small test tubes, in flat glass cells, or in wedge-shaped 

 cells. 2 As each color varies in refrangibility, the focus must be adjusted 

 by the screw B, Fig. 61, according to the part of the spectrum that is 

 examined. When it is desired to see the spectrum of an exceedingly 



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 Mr. 

 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.) Another construction possessing some advantages over the origi- 

 nal form, has been devised by Zeiss of Jena (See "Journ. of Roy. Microsc. Soc.," 

 Vol. iii., 1880, p. 703). 



2 A series of specimens, in small tubes, for the study of Asorption- spectra, is 

 kept on sale by Mr. Browning ; and the directions given in his ' ' How to Work 

 with the Micro-Spectroscope " should be carefully attended to. 



Bright-line Spectre-Micrometer. 



