REVERSED AND NON-REVERSED SPECTRA. 15 



ference is therefore impossible. Again, if the spectrum lines are in coincidence, 

 the longitudinal axes usually diverge by a small angle. Furthermore, the 

 interferences are almost always eccentric and the lines hair-like, indicating 

 distant centers. I have not succeeded in making a perfect adjustment, 

 systematically; but the discrepancies indicated are themselves interesting in 

 their bearing on the subject of this paper. 



In figures 7 and 8, L is a vertical blade of white light from a collimator 

 with fine slit, and G is the grating. The two first-order spectra leaving the 

 ruled face at the line g strike the opaque mirrors M and N, the former on a 

 micrometer moving the mirror parallel to itself. From M and N the rays 

 reach the half -silvered plate of glass HS, where one is transmitted and the 

 other reflected into the telescope T. The coincident rays R are superfluous. 



After placing the parts and roughly adjusting them for symmetry with 

 sunlight, the finer adjustment may be undertaken. It may be noticed that 

 the two systems M and N, and G as well as HS, can be used for further 

 adjustment separately. All are provided with adjustment screws relatively 

 to rectangular axes. To put the mirrors M and N in parallel and in the 

 vertical plane with the grating G, the half-silvered plate should be removed 



/ 



10 



and replaced by a small white vertical screen of cardboard, placed at right 

 angles to the direction of HS in figure 7 and receiving both spectra. A fine 

 wire is drawn across the slit to locate the longitudinal axis, and an extra 

 lens may be added to the collimator and properly spaced until the doublet 

 insures sharp focussing. Both mirrors, M and N, are now rotated on hori- 

 zontal axes, until the longitudinal black lines in their spectra cease to diverge 

 and coincide accurately. G, M, N, may now be considered in adjustment. 

 On returning the half-silvered plate, HS, it in turn is to be carefully rotated 

 around horizontal and vertical axes, until the horizontal black line in the 

 spectrum and the sodium line (always incidentally present in the arc lamp) 

 both coincide. But, as a rule, it will be found that if the longitudinal axes, 

 ww, figure 9, coincide, the D lines cross each other at a small angle, exagger- 

 ated in the figure. The interferences, when found by moving the micrometer 

 at M, are usually coarse, irregular lines, indicating a center not very distant 

 and located on the level of a band where the D lines cross. 



On the other hand, if the D lines are brought to coincidence by moving 

 the adjustment screws on M and N (which throws them out of parallel), the 

 longitudinal axes ww, w'w', figure 10, diverge at a small angle and the inter- 

 ferences are found in a vertical band where the lines ww and w'w' cross. This 

 band is relatively wide, however, as compared with the cases in paragraphs 2 

 and 3. Nevertheless, I have looked upon these results as additional proof 



