xv iii CONTENTS. 



nation in colored rays. Greening of leaves takes place in the yellow 

 and adjacent rays. The essential condition of all chemical changes 

 by radiation is absorption. Plants absorb force from the sun ; it is 

 associated with their combustible parts, and is disengaged by oxida- 

 tion Page 177 



MEMOIR XII. 



EXPERIMENTS TO DETERMINE WHETHER LIGHT PRODUCES ANY MAG- 

 NETIC EFFECTS. 



Mrs. Somerville's experiments. Christie's experiment. Their results 

 cannot be substantiated. The violet ray has no effect. Blue glasses 

 and blue ribbons also ineffective , 191 



MEMOIR XIII. 



AN ACCOUNT OF SOME EXPERIMENTS ON THE LIGHT OF THE SUN, 

 MADE IN THE SOUTH OF VIRGINIA. 



Absorption of luminous radiations. The reference spectrum. Absorp- 

 tion of heat radiations ; the apparatus employed. Absorption of chem- 

 ical radiations. Screen of bromide of silver. Coloration of chloride 

 and bromide of silver by radiations that have passed through corre- 

 spondingly colored solutions. Early application of photography to 

 the investigation of physical problems. Crystallization of camphor to- 

 wards the light. The side of vessels towards the sky is the colder. .197 



MEMOIR XIV. 



EXAMINATION OF THE PROCESS OF DAGUERREOTYPE. NOTE ON 

 LUNAR PHOTOGRAPHY. 



Description of the process. Cause of the deposition of mercury and pro- 

 duction of the light parts of the picture. Polishing of the plate. The 

 operation of iodizing. Effect of keeping the iodide. The achromatic 

 lens. Reduction of focal length in the non-achromatic. The develop- 

 ment. fixing by hyposulphite and galvanism. Necessity of heating 

 the plates. Lunar impressions. Artificial light. Note on lunar 

 photography. The first photographs of the moon 206 



MEMOIR XV. 



ON THE TAKING OF PORTRAITS FROM LIFE BY PHOTOGRAPHY. 



History of the invention. First attempts by whitening the face. Use of 

 refiecting mirrors. Use of a blue-colored trough. Kind of camera 



