ON SPECTRUM MICROSCOPES. 17 



power of quartz ; their observations which are not yet ended go as 

 far as the line R. 



But it is chiefly to the use of this apparatus for astronomical obser- 

 vations that M. Soret wishes to draw attention : the study of the ultra- 

 violet spectrum, in the centre or on the edges of the solar disc, in the 

 protuberances and in the spots, would be most easy with this instru- 

 ment, especially by adapting it to reflecting telescopes ; it would no 

 doubt lead to interesting results. 



ON SPECTRUM MICROSCOPES AND THE MEASURING APPARATUS 

 USED WITH THEM. By H. C. SORBY, F.R.S.; Pres. R.M.S., &c. 



The object of the author was to exhibit and explain the various kinds 

 of apparatus shown in the Exhibition, that had been contrived to 

 examine and measure the spectra of small coloured objects seen under 

 the microscope. The first form was that described by the author in 

 the Quarterly Journal of Science for 1865, vol. ii., p. 198, in which the 

 slit was placed some distance from the microscope, and the prism 

 under the achromatic condenser, so that a small image of a spectrum 

 was seen in focus at the same time as the object on the stage. The 

 characteristic spectrum of the object was then shown by the absorp- 

 tion of particular rays. In this form of apparatus the measurement 

 of the spectra was effected by means of a micrometer placed in the 

 eye-piece. The original apparatus is exhibited. It served very well 

 to prove that a wide field for research would be opened out by the 

 further use of the instrument, but it was soon found to be very incon- 

 venient not to be able to observe the spectra of very imperfectly 

 transparent or opaque substances. This led to the adoption of a 

 spectrum eye-piece, in which the slit is placed in the focus of the eye 

 lens, and compound direct-vision prisms are placed over it. They can 

 thus be taken off, and the object seen through the opened slit, and 

 then on closing up the slit and placing on the prisms the spectrum of 

 the object can be seen. A reflecting prism and side stage enable the 

 observer to compare two spectra together. Eye-pieces of this kind are 

 exhibited by Mr. Browning and Messrs. Becks, and another by Mr. 



C 



