n6 SCIENTIFIC APPARATUS. 



times, through the same train of prisms. In some the greatest 

 effects are sought in wide dispersion ; in others they depend upon 

 high magnifying power ; but the value of each of these varieties 

 will depend upon the purpose to which the instrument is to be 

 devoted. 



It is perhaps hardly necessary to mention here that the spec- 

 troscope has been the instrument with which the great discoveries 

 in chemistry and solar physics have been made within our own 

 time. Among the first investigators in the study of metals by 

 spectrum analysis was the late Sir. C. Wheatstone. The discovery 

 of new metals by the same process has been largely due to Profes- 

 sors Kirchhoff and Bunsen. in Germany ; and Roscoe and Crookes 

 in our own country have added much to our knowledge by the use 

 of the same instrument. While the application of the same method 

 to the great questions of solar and celestial physics has been the 

 subject of the labours of Frankland, Lockyer, and others, in 

 this country ; of Jansen in France ; and of Respighi, Secchi, and 

 others in Italy ; it would be impossible within the present limits 

 to do justice to all the claims of workers in spectroscopy ; but the 

 labours of Rutherford and Draper, in America ; of Jamin and 

 others in France, as well as those of our own countrymen, deserve 

 an honourable mention in connection with the application of 

 photography to the study of the more refrangible part of the solar 

 spectrum. 



The applications of this branch of optics to practical purposes 

 are as yet doubtless in their infancy, but the spectroscope has been 

 applied, not merely to the detection of metals by their bright 

 lines, and of other substances by their absorption bands, but Mr. 

 Lockyer has suggested a process for quantitative analysis by means 

 of measurements of the lengths of the bright lines developed in the 

 spectra of the incandescent vapours of metals under combustion. 



Light is said to be polarised when it presents certain pecu- 

 liarities which it is not generally found to possess. These peculiari- 

 ties, although very varied in their manifestations, have this feature 



