440 EXPERIMENTAL INVESTIGATIONS. 



had been placed, so as to catch the reflected beam, while my 

 assistant alternately made and broke contact with the battery. 

 When the incident sunlight was sufficiently intense to mask 

 the emitted light of incandescence I could not, in the slightest 

 degree, distinguish whether the platinum was ignited or cold. 

 When I first tried it I two or three times complained wrongly 

 to my assistant that he had not made contact when I told him 

 to do so ; when the incident light was very dim, the emitted 

 light was, of course, also distinguishable. I now caused the 

 spectrum from a flint-glass prism to fall, on the platinum, and 

 with similar effect ; i.e. when the reflected spectrum was very 

 intense, no difference could be detected between the light from 

 the platinum, whether cold or ignited, or whether received 

 upon paper or upon the eye ; when less intense, the red por- 

 tion of the spectrum was elongated by the light of incandes- 

 cence, and the other portions partook of the character of the 

 spectrum superposed upon, or blended with, the light of incan- 

 descence. 



The prism was also arranged so as to intercept the reflected 

 instead of the incident beam ; the effects were similar. 



A beam of light polarised by reflection at the proper angle 

 from a plate of glass was made to fall on the platinum surface, 

 and then analysed by a tourmaline ; no difference was per- 

 ceptible in the plane of polarisation, whether the platinum 

 was ignited or not. 



The light reflected from the platinum was similarly polar- 

 ised and analysed, but no difference dependent upon incan- 

 descence was detected. 



A wire of platinum, 6 inches long and -^th of an inch in 

 diameter, was vertically suspended in the narrow fissure of the 

 shutter ; the bands of interference were received on paper 

 placed at different distances from the wire, and examined 

 both by the eye and by a lens ; no difference could be de- 

 tected in these bands when the wire was ignited by a voltaic 

 battery. 



In all the above experiments the foil or wire was ignited 

 by the battery previously to the commencement of each class 

 of experiment, so as to avoid any effect arising from the altera- 

 tions caused by the platinum having been subjected to heat, 



