492 C. Barus — Interferences of Grossed Spectra. 



white light is here furnished by the collimator L with a 

 horizontal slit and the rulings of the grating are also hori- 

 zontal and parallel to it), twice in succession and preferably 

 from mirrors M and N and m and n, reflecting normally to 

 each other and inclined at an angle of roughly 45°. Each of 

 the mirrors M and iV^must be revolvable about a horizontal axis 

 parallel to the slit and furnished with three adjustment screws 

 relatively to axes normal to each other, one of which is 

 horizontal. The mirrors m, n are the silvered faces of a prism 

 right-angled at the edge. It is moreover to be placed on the 

 slide of a Fraunhofer micrometer so that the prism may be 

 moved, gradually, up and down for the adjustment of distances. 

 On leaving the mirror m, n, the two spectra are carried by 

 nearly horizontal and parallel sheets of divergent rays, which 

 pass outward from the diagram. But it will be seen that one 

 of the two spectra reaching the observer is reversed on the 

 longitudinal axis relatively to the other ; i. e. if one is in the 



position red | ^ Qm | violet, ^j^ red j B ^ m } violet. 



The subsequent passage of the rajs is shown in figure 3, b, 

 which is the side elevation and therefore at right angles to the 

 preceding figure. The rays from m and n impinge on a 

 distant slightly concave mirror K (about 1*7 meters in focal 

 distance) placed somewhat obliquely, so that when the rays come 

 to a focus at F near the micrometer, they may just avoid it. 

 The partially overlapping spectra at F are viewed by a 

 strong eyepiece F. The observer at F can then control the 

 Fraunhofer micrometer by which on, n is raised and lowered, 

 and the three adjustment screws of M. 



The adjustment consists in first roughly placing all parts in 

 symmetry with sunlight, until the two spectra appear at F. 

 The lens may be removed. There should be a bright narrow 

 spectrum band on each side of and near the edge of the prism 

 m n ; for it is clear that after passing the lens F, corresponding 

 rays from M and iVmust both enter the pupil of the eye to be 

 seen together. To make the spectrum parallel, the mirror 

 m n is rotated, as a whole, around a vertical axis. The three 

 screws on the mirrors M and JV then assist in complet- 

 ing the adjustment ; the rotation around the horizontal axis 

 brings the sodium lines in coincidence (both must be clearly 

 seen and sharp and at an appreciable distance apart) ; that 

 around the oblique axis gives rise to more or less overlapping, 

 as required. The need of a sharp coincidence of the sodium 

 lines is very essential in all these experiments. 



After proper vertical position of m n has been found by 

 slowly moving the micrometer screw up and down, the fringes 

 appear. They are usually very fine lines, indicating distant 



