Barus — Use of the Grating in Interferometry . 163 



with a sharp spectrum. Fortnnately many positions are avail- 

 able for obtainin<>; the ellipses, so that a satisfactory one is 

 easily found. Unless all overlapping spectra show sharp lines 

 the adjustment is very tedious. The amount of grating space 

 is less than 1 sq. cm., though, of course, a larger size is con- 

 venient for experiment. 



2. Special properties. — Tjie motion of the ellipses bodily 

 across the spectrum, corresponding to the increased or decreased 

 virtual air space (micrometer screw), shows that whereas the 

 vertical dimension or axes (direction of fixed color) do not 

 appreciably change,* the horizontal axes grow rapidly smaller ; 

 i. e., the ellipse is more eccentric from red to violet. With the 

 grating used, however (about 2800 lines per cm.), the major 

 axis remained vertical, i. e. the circular form was not reached 

 throughout the first order even in extreme red. 



It follows from this that in the second order the major axis 

 of the ellipses will probably be horizontal, and this was found 

 to be strikingly true. The fignre, moreover, is necessarily 

 coarser and the lights aiid shadows in greater contrast. In the 

 third order the ellipses are drawn out horizontally to a corre- 

 spondingly greater degree. Thus it is clear that with a more 

 dispersive grating, the circular or even the horizontally elon- 

 gated form must occur in the first order. Spectrum lines are 

 very distinct, particularly in the second order. In all orders 

 of spectra the centers of the ellipses are simultaneously on the 

 same spectrum line and their vertical dimensions are about the 

 same. Hence the spectrum lines may be used instead of cross 

 hairs as they are fixed landmarks among the moving ellipses. 



With replica gratings the center of the ellipses is usually 

 remote ; i. e., reflection does not easily take place at the grating 

 surface. Apart from this the curved lines are good and strong. 

 Samples must be tried out, in which the lines are as clear as 

 possible in all the overlapping spectra. Six gratings of this 

 kind were examined with about the same results. The centers 

 of the ellipses could only rarely be brought into the field. 



It has been stated that considerable width of slit is admis- 

 sible. Moreover the focal plane of the collimator (convergent 

 or divergent) light or the focus of the telescope makes little 

 difference, though the condition of parallel rays is naturally 

 preferable. An eye focused for infinity sees the fringes very 

 well without the telescope and they may be also caught on a 

 screen ; but coming through a slit they are liable to be dark 

 and the advantage of the telescope is obvious. The second 

 order is particularly accessible for naked-eye observation, and 

 the light and black appearance is accentuated, but they are 



* Probably referable to increased refraction and decreased diffraction from 

 red to violet. 



