80 On a new Form of Spectrometer. 



modifying the apparatus so as to use a grating in place of the 

 prism. The observations in this case of sunlight were quite 

 as satisfactory as those in which gaslight had been used. 



General Conclusions. 



1st. In the prismatic spectrum the luminous intensity in- 

 creases from the more to the less refrangible spaces, its 

 maximum being not in the yellow but in the red. This is due 

 to the action of the prism, which narrows and as it were con- 

 denses the coloured spaces more and more as we pass towards 

 the red, increasing the intensity of the light as it does that of 

 the heat. 



2nd. In the grating or diffraction-spectrum the luminous 

 intensity is equal in all the visible regions, all the colours 

 being simultaneously obliterated by an extinguishing light. 



It must however be borne in mind that these conclusions 

 should be taken in connexion with the physiological action of 

 the eye. Owing in part to the imperfect transparency of its 

 media, and partly to the inability of its nervous mechanism to 

 transmit waves of certain frequency to the brain, the spectrum 

 does not begin and end sharply, as to a perfect eye a perfect 

 spectrum ought to do. 



There are hence two causes which must not be overlooked 

 in these observations. 1st, the physiological peculiarity of 

 the eye, which gives to each end of the spectrum the aspect 

 of gradually fading away; 2nd, in the case of solar light 

 the absorption action of the atmosphere, which is chiefly ex- 

 erted on the more refrangible rays. 



I think, bearing in mind the correlation of light and heat, 

 both being corresponding manifestations of the same vibrating 

 movement in the ysther, that these results substantiate those I 

 published in 1872 on the distribution of heat in the spectrum, 

 and that as the different coloured spaces are equally luminous, 

 so they are equally warm. 



I have made some attempt to compare with each other the 

 luminous intensity of the bright lines in various spectra, 

 especially those emitted by a strontium-flame ; but not being 

 able to continue these researches at present, I have postponed 

 them to a more favourable opportunity. 



University of New York, 

 May 5th, 1879. 



