THE AID OF THE ACHROMATIC FRINGES. 



101 



like the effect of variation in color when thickness is constant (linear phenom- 

 enon of reversed spectra) . The pattern in the last case is of course apt to be 

 enormously finer or narrower. From this point of view the enlargement of the 

 linear phenomenon may have a direct bearing on the question of interference 

 of light of slightly different wave-length. 



When produced by a single dispersion the linear phenomenon is independent 

 of the amount of dispersion, being alike in width for the spectra of a 30 prism 

 and of a strong grating, for instance. It depends, therefore, for its width on 

 the diffraction of the telescopic system. Thus, if one decreases the aperture 

 of the observing telescope by a circular screen, the linear phenomenon in- 

 creases in width with the loss of resolving power. When the linear phenom- 

 enon is viewed through a spectro-telescope (i.e., subjected to a second inde- 

 pendent dispersion) this is no longer the case. The phenomenon broadens 

 and shows much more variety of detail, even though there is liable to be 

 deficiency of light. 



It is necessary that the original phenomenon shall be symmetric (arrow- 

 heads, or closely packed very eccentric ellipses), otherwise the enlargement 

 merely brings out an awned structure difficult to interpret. The symmetrical 

 linear phenomenon enlarged by the second dispersion broadens out so that the 

 fine lines from each arrow-head may be seen to about five times the width of 

 the sodium lines, on either side of the apices. On widening the slit only those 

 parts which are parallel to the second plane of dispersion are accentuated. 

 Hence, if this is horizontal, the broad-slit phenomenon is bead-like in struc- 

 ture. For oblique planes it becomes more and more linear. Fine fringes may 

 thus be detected. 



With the non-symmetric linear phenomenon, the enlargement resulting 

 from the second dispersion sometimes brings out the arrow forms on widening 

 the slit. One or the other side is cut off on closing the slit. 



77. Monochromator. The use of two identical direct-vision gratings (prism 

 gratings) for the purpose of obtaining approximately homogeneous light is not 

 only very convenient but has certain ulterior advantages, provided the light 

 is not deficient. Each of these consists of a 

 cap C (fig. 100, front view; fig. 101, sectional 

 plan) fitting the end of a telescope T like an 

 ordinary cap, but provided with a plate rr in 

 front, to which the swiveling plate t is attached 

 by a spring and bolt at q and a stop at s. The 

 plate t carries the grating g and prism p for 



00 



direct vision. When not wanted the plate t is rotated on q to one side. The 

 spring keeps the plate in any position when the telescope T (and with it the 

 grating g) is rotated on its axis. 



One of these gratings is to be attached at the collimator (grating I) and the 

 other at the telescope (grating II). It will be seen that if the gratings are 

 similarly oriented the dispersion is summational (Di-\-D 2 ); when either is 



