XVI 



CONTENTS. 



MEMOIR IY. 



ON THE NATURE OF FLAME AND ON THE CONDITION OF THE SUN 5 S 



SURFACE. 



Dove's experiments on electric light. Dark lines replaced by bright ones. 

 Electric spark between metallic surfaces. The lines depend on the 

 chemical nature of the substance from which the light issues. They 

 may be used for determining the physical condition of the sun and 

 stars. Three hypotheses of the condition of the sun's surface exam- 

 ined Page 81 



MEMOIR Y. 



ON THE NEGATIVE OK PROTECTING RAYS OF THE SUN. 



Original discovery of protecting radiations. Case of a daguerreotype 

 plate. Spectrum -photographs made in Virginia. Protecting action 

 of the less refrangible rays. Protecting action of the extreme violet. 

 Variations of the protecting action. Spectrum of darkness and spec- 

 trum of daylight. Interference of rays of different colors. Action of 

 waves of red, yellow, and violet light with ivave-lengths 2, 1^, 1. Use 

 of the diffraction spectrum 87 



MEMOIR VI. 



ON THE DIFFRACTION SPECTRUM. 



Mode of obtaining the diffraction spectrum. The yellow is in its mid- 

 dle ; it is a centre of chemical action. It is also the hottest ray. 

 Diffraction spectra by reflection. Cold lines. Dilatation of the more 

 refrangible rays in the prismatic spectrum, compression of the less re- 

 frangible. Action of the diffraction spectrum on salts of silver. Use 

 of wave-lengths for spectrum division 97 



MEMOIR VII. 



STUDIES IN THE DIFFRACTION SPECTRUM. 



Elementary description of the diffraction spectrum. Young's discovery 

 of interference. FresneVs discovery of transverse vibrations. Gratings 

 and the spectra they yield. Gratings on reflecting surfaces. Optical 

 action of a grating. Its spectra of different orders. Interpretation 

 of wave-lengths by the mind. Mental appreciation of multiple wave- 

 lengths. Refutation of the principle that to every color there belongs 

 an invariable wave-length. Increase in the range of perception in the 



