C. Barus — Use of Compensators. 305 



In conclusion, therefore, the main feature in modifying the 

 type of interference pattern is the varying thickness of the 

 compensator. For oval types the preponderating lens is con- 

 vex, for the hyperbolic type it is concave. Neither of these 

 lenses are here appreciably affected in modifying the horizontal 

 distribution of path difference because the dispersion of the 

 grating requires a horizontally parallel system of rays. 



6. Telescopic interferences. — If interference patterns of 

 small angular extent are to be obtained, it is essential that the 

 rate at which obliquity increases from ray to ray be made as 

 large as practicable. Probably, therefore, an opportunity for 

 realizing these conditions may 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 in- 

 terference. 



The experiment was made on the long interferometer, fig. 4, 

 the distances between mirror M and grating G and from the 

 latter to the mirror N being nearly two meters each. L 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 0', which is revolvable about the ver- 

 tical, is thick enough to exactly counterbalance, if necessary, 

 the thickness of the glass plate of the grating and of the lens 

 system L. 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 latter may be removed. 



The telescope directed along the axis R will, therefore, in 

 general see two white slit images, A and A', fig. 5, not both in 

 focus at once ; A coming from M being larger, A' from JV 

 (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, Z, is larger. 

 Similarly the two spectra are observed along the 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 prin- 

 cipal focal plane, containing 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 l cm . The focal planes of the 



