CHAPTER II. 



FURTHER STUDY OF THE INTERFERENCE OF REVERSED SPECTRA. 



7. Apparatus with one grating. The different methods suggested in para- 

 graph 3 were each tried in succession, but none of them were found equally 

 convenient or efficient in comparison with the method finally used in the 

 preceding paper. To begin with the annoyances encountered in the use of a 

 reflecting grating, it was found that the impinging light from the collimator 

 and the reflected doubly diffracted beam from the grating lie too close together, 

 even if all precautions are taken, to make this method of practical value. The 

 use of Rowland's concave grating without a collimator is out of the question, 

 since the spectra formed on the circular locus of condensation, if reflected 

 back, will again converge into a white image of the slit, colored if part of the 

 spectrum is reflected. The plane-reflecting grating, though not subject to 

 this law, requires a collimator, and, since marked obliquity of rays is excluded, 

 it will hardly be probable that the elusive phenomena can be obtained in this 

 way. A compromise method, in which both the reflecting and the transmitting 

 grating are used, will be described in paragraph 10. Though apparently the 

 best adapted of all the methods used, it has only after difficult and prolonged 

 research led to results. These, however, proved very fruitful in their bearing 

 on the phenomena. 



For first-order spectra, where there is abundance of light (it is often difficult 

 to exclude all the whitish glare in the field of the telescope completely), the 

 method of figure n, which shows normal rays only, is still preferable. Here 

 the impinging collimated beam L passes below the opaque mirror m and 

 through the lower half of the grating G. The diffracted pencil is reflected 

 nearly normally but slightly upward, by the mirrors M and N (the former 

 carried on a micrometer slide) , to be again diffracted at the grating and there- 

 fore to impinge as definitely colored light on the lower edge of the concave 

 mirror m (about 1.5 to 2 meters in focal distance), whence it is brought to a 

 focus at F and viewed by the strong eyepiece E. Considerable dispersion 

 and magnification is obtained in this way; indeed, the two D lines stand far 

 apart and the nickel line is distinctly visible between them. There must be 

 a fine hair wire across the slit so that the longitudinal axes of the spectra may 

 be accurately adjusted. The mirror m above the impinging beam must be 

 capable of rotation about a vertical and a horizontal axis in order that the 

 focus F may be appropriately placed between M and N. With G at i meter 

 and m at 2 meters from F, the disposition is good. The micrometer M is 

 easily at hand. Though the direct beam may be screened off, the glare 

 reflected back from the grating and the glare from the objective of the colli- 

 mator are not excluded, as stated. In fact, it was eventually found necessary 



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