.John WILLIAM DBAPBB. 



Hence it follows that any two series of undulations in the spectrum 

 will have the same heating power, no matter what their wave- 

 lengths may he. 



Continuing his spectrum investigations, Dr. Draper next sought 

 to determine the law of the distribution of light Mosotti, in Italy, 

 had already studied I In- diffraction spectrum, and had shown that 

 the maximum of illuminating power lay in the yellow, the intensity 

 declining symmetrically on either side. The photometric method 

 of Bouguer Dr. Draper had used in 1*47, with good results, to 

 measure the intensity of the light radiated from incandescing plat- 

 inum ; and he now, in 1879, sought to construct a spectrometer on 

 the same principle which should measure light-intensity. The prin- 

 ciple is a well-recognized one in optics. It is that any light becomes 

 invisible in presence of another light sixty four time- as strong. If, 

 as Mosotti had maintained, the yellow be the brightest of the spec- 

 trum colors, then in presence of an extraneous light thrown on 

 the spectrum and variable at will, the yellow will remain after the 

 red and orange on the one side, and the green, blue, and violet on 

 the other, have been extinguished. But onmakingthe experiment, 

 throwing the extinguishing white light through the third telescope 

 of an ordinary spectroscope, so as to reflect it to the eye from the 

 face of the prism, Dr. Draper found that the colors of a gas-flame 

 spectrum disappeared in the inverse order of their refrangibility, 

 the red being the last to disappear. Analogous experiments with 

 the spectrum of sunlight, winch was sometimes thrown on a screen 

 and sometimes on the ground glass of a camera and extinguished 

 by daylight, gave precisely similar results. On gradually opening 

 the shutter admitting the daylight, the extreme violet disappeared 

 first, and then the other colors in the inverse order of refrangibility 

 as before. On closing the shutter the red first came iuto view, and 

 then the other colors successively. Obviously, if this result is due 

 to the compression at the red end of the prismatic spectrum, then it 

 should not be found in that produced by diffraction; and on mak- 

 ing the experiment Dr. Draper saw, not without pleasure, that as 

 the intensity of the extinguishing beam increased all the colored 

 spaces yielded apparently in an equal manner and disappeared at 

 the same moment. On diminishing the intensity of the extraneous 

 light he observed that they all came into view apparently at the 

 same time. The yellow, as before, showed no superiority over the 

 other colors in resisting extinction. It would seem, therefore, that 



868 



