120 



THE INTERFEROMETRY OF 



usual way with the motion of the micrometer and indicating a center of ellipses 

 very distant in the field of the spectrum. In other words, the interference 

 pattern is no longer automatically centered and is therefore useless. 



A modification of this plan is the method shown in figure 86 (horizontal 

 section), where B is the beam from the collimator, L, L',L",L"', four condens- 

 ing lenses of the same power (/= 50 cm.), G the grating, Mand N opaque plane 

 mirrors, T the telescope. In all the above cases the horizontal rays from the 

 collimator traverse the grating in parallel and eventually condense to a single 

 point in the field of the telescope. The same is true of all rays having the same 

 angle of altitude. These rays, therefore, act as a whole, since they pass through 

 the plate of the grating at the same angle of incidence. On the other hand, 



85 



86 



relative to a vertical plane, the rays traverse the grating at different angles, 

 each angle corresponding to a horizontal strip of the spectrum. It is by the 

 easy modification of this obliquity that the curved compensator becomes effec- 

 tive. In figure 86 the rays are also oblique relative to a horizontal plane; but 

 the result, unfortunately, is not available, since each of these oblique rays must 

 have its own complete spectrum. Consequently the diffracted pencil will con- 

 sist of an infinite number of overlapping spectra, the extreme cases lying within 

 the same angle a shown in the figure. A large telescopic objective would then 

 reunite these spectra into a white image of the slit, while a small objective will 

 show colored slit images, passing from impure red to impure violet. Naturally, 

 the interferences will also overlap, and therefore vanish. 



69. 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, 



