REVERSED AND NON-REVERSED SPECTRA. 121 



an opportunity for realizing these conditions will be found within the telescope ; 

 i.e., after the rays pass the objective. The endeavor would therefore be 

 directed to bringing two spectra, focussed in two planes, one of which is behind 

 the other and consequently of different sizes, both vertically and horizontally, 

 to eventual interference. 



The experiment was made on the long interferometer (fig. 87), the distances 

 between mirror M and grating G and from the latter to the mirror N being 

 nearly 2 meters each. C is the lenticular compensator, consisting of two lenses, 

 respectively concave and convex, each having the same focal distance, /= 50 

 cm. The distances apart, D, of the lenses may be varied. The glass plate C', 

 which is revolvable about the vertical, is thick enough to exactly counterbal- 

 ance, if necessary, the thickness of the glass plate of the grating and of the lens 

 system C. A sharp wedge sliding transversely may also be used. It is best to 

 replace C' by two plates of glass, one thick and the other thin, so that the lat- 

 ter may be removed. 



The telescope directed along the axis R will therefore, in general, see two 

 white slit images, A and A' (fig.88), not both in focus at once, A' coming from 



cA- n a' 

 a 



"6 f 



C' 



C 



87 88 



M being larger, A from N (parallel rays) smaller. The focal plane of A' will be 

 towards the grating as compared with A , and A' is larger than A , in proportion 

 as the distance apart of the lenses C is larger. Similarly, the two spectra 

 are observed along the diffraction axis, D, not in focus at once and of 

 different areas. 



To obtain the interferences the slit image A must be placed anywhere within 

 A', and they will occur at the top of the spectrum if a and a' are vertically in 

 coincidence; in the middle if b and b' coincide, etc. 



The plane of the new interferences is no longer the principal focal plane, con- 

 taining the Fraunhofer lines, but lies in front of it; i.e., towards the eye of the 

 observer and away from the grating. This distance, measured along D for the 

 given small telescope used, was fully i cm. The focal planes of the two spectra 

 are usually not so far apart. A' corresponds to a virtual object behind the 

 observer. 



If the vertical plane in which the interferences lie be taken as the image, the 

 object would be situated about 3 meters beyond the objective of the telescope 

 used. This would place it 30 cm. in front of the mirror M or N, where there is 

 but a single beam in each case. In fact, the telescope may be brought quite 



