96 PLEASANT WA YS IN SCIENCE. 



thenceforth, as all the world knows, that the earth is ex- 

 tended instead of flattened at the poles. 



Huggins tried to recognize by the new method the 

 effects of the sun's rotation, using a much more powerful 

 spectroscope than Secchi's. The history of the particular 

 spectroscope he employed is in one respect specially in- 

 teresting to myself, as the extension of spectroscopic power 

 was of my own devising before I had ever used or even 

 seen a powerful spectroscope. The reader is aware that 

 spectroscopes derive their light-sifting power from the prisms 

 forming them. The number of prisms was gradually in 

 creased, from Newton's single prism to Fraunhofer*s pair, 

 and to KirchhofPs battery of four, till six were used, 

 which bent the light round as far as it would go. Then 

 the idea occurred of carrying the light to a higher level 

 (by reflections) and sending it back through the same 

 battery of prisms, doubling the dispersion. Such a battery, 

 if of six prisms, would spread the spectral colours twice 

 as widely apart as six used in the ordinary way, and would 

 thus have a dispersive power of twelve prisms. It occurred 

 to me that after taking the rays through six prisms, arranged 

 in a curve like the letter C, an intermediate four-cornered 

 prism of a particular shape (which I determined) might be 

 made to send the rays into another battery of six prisms, the 

 entire set forming a double curve like the letter S, the rays 

 being then carried to a higher level and back through the 

 double battery. In this way a dispersive power of nineteen 

 prisms could be secured My friend, Mr. Browning, the 

 eminent optician, made a double battery of this kind,* 



I have omitted all reference to details ; but in reality the double 

 battery was automatic, the motion of the observing telescope, as 

 different colours of the spectrum were brought into view, setting all the 

 prisms of the double battery into that precise position which causes them 

 to show best each particular part of the spectrum thus brought into 

 view. It is rather singular that the first view I ever had of the solar 

 prominences, was obtained (at Dr. Huggins's observatory) with this 

 instrument of my own invention, which also was the first powerful 

 spectroscope I had ever used or even seen. 



