414 C. Bar us — Interference of Reversed Spectra. 



Art. XXX. — Further Study of the Interference of Reversed 

 Spectra ; by Cael Barus.* 



1. Apparatus with one grating. — The different methods 

 suggested in the preceding papersf were each tried in succession, 

 but none of them were found equally convenient or efficient in 

 comparison with the method there finally used. To begin with 

 the annoyance 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 proba- 

 ble 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 § 4. 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 fig. 1, which shows 

 normal rays only, is still preferable. Here the impinging colli- 

 mated beam L passes below the opaque mirror m and through 

 the lower half of the grating (x. 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 therefore to impinge as 

 definitely colored light on the lower edge of the concave mirror 

 m (about 1*5 to 3 meters in focal distance), whence it is brought 

 to a focus at F and viewed by the strong eyepiece F. Con- 

 siderable dispersion and magnification is obtained in this way ; 

 indeed, the two D lines stand far apart and the nickel line is 

 distinctly Tisible 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 imping- 

 ing beam must be capable of rotation about a vertical and a 



* Abridged from a forthcoming Eeport to the Carnegie Institution of 

 Washington, D. C. 



t This Journal, xl, pp. 486-498, 1915. 



