•±6 Messrs. 0. and M. Barus on the Interference of 



Granting that the fringes in question depend upon the 

 reflecting surface behind the grating, they must move if the 

 distance between them is varied. Consequently a phenomenon 

 so easily produced and controlled is of much greater interest 

 in relation to micrometric measurements than at first appears, 

 and we have for this reason given, it detailed treatment. It 

 has the great advantage of not needing monochromatic light 

 and of being applicable for any wave-length whatever, and 

 admitting of the measurement of small horizontal angles. 



When the phenomenon as a whole is carefully studied it is 

 found to be multiple in character. In each order of spectrum 

 there are different groups of fringes of different angular sizes 

 and usually in very different focal planes. Some of these are 

 associated with parallel light, others with divergent or con- 

 vergent light, so that a telescope is essential to bring out the 

 successive groups in their entirety. At any deviation the 

 diffracted light is necessarily monochromatic ; but the fringes 

 need not and rarely do appear in focus with the solar spectrum. 

 If the slit of the spectroscope is purposely slightly inclined 

 to the lines of the grating certain of the fringes may appear 

 inclined in one way and others in the opposite way, producing 

 a cross pattern like a pantograph. The reason for this 

 appears in the equations. 



In any case the final evidence is given when the reflecting 

 face behind the grating is movable parallel to it. The inter- 

 ferometer so obtained is subject to the equation (air space e, 

 wave-length \ 7 angle of incidence i, of diffraction #'), 



Be =X/2(cos 0' — cos i), 



and is therefore less unique as an absolute instrument than 

 Michelson's classic apparatus or the device of Fabry and 

 Perot. Its sensitiveness per fringe, Be, depends essentially 

 upon the angle of incidence and diffraction and it admits of 

 but 1 cm. (about) of air space between grating face and 

 mirror before the fringes become too fine to be available. 

 But on the other hand it does not require monochromatic 

 light (a Welsbach burner suffices), it does not require optical 

 plate glass, it is sufficient to use but a square cm. of grating 

 film, and it admits of very easy manipulation, for painstaking 

 adjustments as to normality, &c, are superfluous. In fact, it 

 is only necessary to put the sodium lines in the spectrum 

 reflected from the grating and from the mirror into coin- 

 cidence both horizontally and vertically with the usual three 

 adjustment screws on grating and mirror. Naturally sun- 

 light is here desirable. Thereupon the fringes will usually 

 appear and may be sharply adjusted upon a second trial 

 at once. 



