818 



HANDBOOK OF PHOTOGRAPHY 



standard lines on the same plate, care being taken to allow no sideways displacement 

 of the plate between exposures. Even the racking of a plate from one position to 

 another will involve some sidewise displacement, and the two spectra should be taken 

 with no intervening motion of the plate when precision is required. Diaphragms may 

 be inserted to shorten the lengths of some of the lines, as in Fig. 10, so that lines of the 

 two spectra can be separated readily. 



The positions of the lines are usually determined with a wavelength comparator, 

 which consists of a carriage to which the plate can be clamped, an observing microscope 

 provided with a cross hair, and a precision screw which moves the plate (or less con- 

 veniently, the microscope itself). Comparators can seldom be read more precisely 

 than to + 0.001 mm., which is sufficient for most spectrograms. Care should be taken 

 to avoid undue magnification in the comparator eyepiece, as unless the spectrum 



Fig. 11. — Identification of unknown spectrum wavelength by means of known spectrum. 



lines are extremely sharp, they can be set most accurately on the cross hair when 

 magnifications of from 5 to 15 diameters are used. In some comparators the spectrum 

 is projected on a screen for measurement. 



For routine identification of lines precision to 0.01 mm. often suffices, in which 

 case a simple plate magnifier with engraved scale will serve. These can be obtained 

 with scales 20 mm. long, engraved with 0.1-mm. divisions which can be estimated to 

 0.01 mm. 



Knowing the wavelength of anj^ one line, the distance to an unknown line can be 

 measured with the eyepiece or comparator. Multiplying this distance by the approxi- 

 mate dispersion of the plate at that point gives the wavelength of the unknown line. 

 For example, in Fig. 11 line A was identified as 3542.079 of iron. The distance to 

 line B was measured as 9.972 mm. The dispersion between lines 'A and C was 

 3542.079 - 3521.264(20.815A.), divided by 14.820 mm. or 1.403 A. per mm. Multi- 

 plying the distance AB by 1.403, we obtain 14.006 A. from A to B, which subtracted 

 from 3542.079 A. for A gives 3528.073 A. for the wavelength of B. 



