100 THE INTERFEROMETRY OF 



tion if the ocular is drawn out, or the reverse if it is thrust in. On close 

 examination two sets, in different focal planes, seemed to be present, one 

 stationary and the other moving as described, and accounting for the observed 

 pronounced parallax. Suggestions of movable fringes accompanying the 

 stationary are also present when the latter are produced by the grating. In 

 this case the stationary fringes are strong ; in the case of simple diffraction the 

 movable fringes are more prominent. 



50. Inferences. -There can be no doubt that the great variety of chan- 

 neled spectra obtained, when white light is successively diffracted by two 

 gratings, is referable to the fringes obtained in the diffraction of homogeneous 

 light, observed outside the principal focal plane, on a spectrometer. In other 

 words, if light of a given pure color (sodium, mercury) is used, a single grating 

 suffices. Each line of the spectrum is resolved into well-defined groups of 

 fringes, if it is observed either in front of or behind the principal focal plane. 

 The arrangement of fringes varies in marked degree with the distance of the 

 plane observed from the latter (x, fig. 71). If reflecting gratings are used, 

 there is no other possible source of interferences ; but reflecting and transmit- 

 ting gratings show the phenomenon equally well. 



After finding how easily the Fresnellian interferences of two virtual slits 

 could be reproduced in the telescope (Chapter III) and observed on either 

 side of (before or behind) the sharp images, it seemed reasonable to suppose 

 that the diffraction of a slit could also be produced and exhibited in this way ; 

 but the availability of this anticipation is attended with much greater diffi- 

 culty. The image of a very distant slit does indeed show separated diffraction 

 fringes on either side of the principal focal plane in the observing telescope. 

 But they move right and left with the eye, in the same direction if the ocular 

 is drawn outward from the principal focal plane, and in the direction opposite 

 to the eye if the ocular is thrust in. Hence, in this respect, the fringes do not 

 at once recall the phenomena under consideration. Usually the blurred image, 

 out of focus, is stringy, without definite structure. It is resolved in a single 

 focal plane only. 



To obtain sharp stationary fringes from an image of the slit, this image must 

 be produced by the diffraction of a grating having a dispersing power above a 

 certain minimum. Thus in a grating of about 7,000 lines to the inch the un- 

 deviated slit image and the image of the first order are not clearly resolved, 

 unless the slit is very fine. In the second and higher orders, however, the res- 

 olution is very pronounced and the fringes stationary. 



The resolution of fringes is equally manifest in front of or behind the prin- 

 cipal focal plane, so that if a weak convex lens is added to the objective of the 

 telescope, the succession of fringes is found with an outgoing ocular; if a weak 

 concave lens is added to the objective, the succession is found with an ingoing 

 ocular, starting in each case near the principal focus. As the fringes increase 

 in size they in turn subdivide, sometimes irregularly, as if each fringe were a 

 new slit image, capable of undergoing secondary diffraction. Beyond these 

 secondary fringes no further resolution was detected. 



