MICRO-SPECTROSCOPIC MAPS. 257 



A centimetre will be found a convenient measure to 

 represent one revolution: the millimetre will then equal 

 ten of the smallest divisions of the milled head, or T ^ of a 

 revolution; these smallest divisions, or thousandths, need 

 not be regarded, as without them the spectrum will be 

 divided with sufficient accuracy. Next measure the dis- 

 tances between the Frauenhofer lines. This should be done 

 with great care, as when once determined it will be constant. 

 The lines A and a need not be measured, as they are in 

 the dark part of the red, and are only to be seen in an 

 extremely bright light, so will hardly be required in prac- 

 tice. The same remark will apply to the line H. The 

 spectrum to be measured will therefore extend from about 

 B to a little beyond G. 



Let this scale be drawn on cardboard and preserved for 

 reference. Now measure the position of the dark bands in 

 any absorption spectra, taking care for this purpose to use 

 lamplight, as daylight will give, of course, the Frauenhofer 

 lines, and tend to confuse the spectrum. If the few lines 

 occurring in most absorption spectra be now drawn to 

 the same scale as the solar spectrum, on placing the scales 

 side by side, a glance will show the exact position of the 

 bands in the spectrum relatively to the Frauenhofer lines, 

 which thus treated form a natural and unchangeable scale. 

 But for purposes of comparison it will be found sufficient to 

 compare the two lists of numbers representing the micro- 

 metric measures, simply exchanging copies of the scale 

 of Frauenhofer lines, or the numbers representing them 

 will enable observers at a distance from each other to com- 

 pare their results, or even to work simultaneously on the 

 same subject. 



Owing to the sliding and other fittings between the 

 prisms and the slit, the micrometer cannot be depended on 

 for pointing to the position of a line by setting it to the 



s 



