MEMOIR VII.] STUDIES IN THE DIFFRACTION bUECTliUM. 



MEMOIR VII. 



STUDIES IN THE DIFFRACTION SPECTRUM. 

 A popular exposition condensed from Harper's New Monthly Magazine, Vol. LV. 



CONTENTS : Elementary description of the diffraction spectrum. 

 Young's discovery of interference. FresneCs discovery of transverse 

 vibrations. Gratings and the spectra they yield. Gratings on reflect- 

 ing surfaces. Optical action of a grating. Its spectra of different 

 orders. Interpretation of wave-lengths by the mind. Mental appreci- 

 ation 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 eye. Extension to photographic impressions. 

 Encroachment on the first dark space. Photographs of the diffrac- 

 tion spectrum. Proposal to use wave-lengths for spectrum divisions. 

 Replacement of imaginary imponderable principles by wave-lengths. 



WHAT is a diffraction spectrum ? Every person who 

 has read a book on light is familiar with the prismatic 

 spectrum, in the study of which Newton displayed his 

 transcendent philosophical powers. The diffraction I 

 have had occasion to refer to several times, and since it 

 is less known, will now describe it. Some very curious 

 phenomena connected with it I have personally exam- 

 ined. It carries us to a true interpretation of the rela- 

 tions of heat, light, and actinism; it offers some impor- 

 tant suggestions respecting the mode of action of that 

 most wonderful of all organs, the brain, and therefore 

 commends itself to our most earnest attention. 



If we look at a candle flame placed ten or a dozen feet 

 distant, the eyelids being so nearly closed that the eye- 

 lashes intercept the incoming rays, we see on either side 

 of the true image of the flame a succession of colored 

 ones rainbow streaks or fringes, as it were. Examining 



