MICROSCOPES. 339 



focus. In both forms of apparatus the spectra of two different 

 objects can be compared together, side by side. This is a most 

 important requisite, though the means of doing it has occasion- 

 ally been omitted in some forms of apparatus. In addition to the 

 essential spectrum part of the instrument, various accessory con- 

 trivances have been made to enable an observer to accurately 

 measure the position of any particular part of the spectrum, and 

 to determine the wave-length of the light at each part. 



H. C. SORBY. 



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