78 DISPLACEMENT INTERFEROMETRY BY 



as observed with 3 diopter concave lenses between m and m', by an ordinary 

 telescope without additional objectives. The slit-images being quite out of 

 focus, the field is uniformly illuminated. Through it pass the succession of 

 forms, 6, a, c (as described), when (for instance) m is rotated slowly on a 

 vertical axis. Many of the forms are quite visible to the naked eye. If the 

 fringes are moderately fine, complete hair-like circles may be produced at 

 b and c. Moreover, beginning with the symmetrical position of the lens, the 

 rays 2 j and 8 7 need not be coincident. 



If the lens is not symmetrical in form, i.e., for plano-convex meniscus and 

 other lenses, the simple figures above discussed become more complicated 

 and the fringes multi-annular. 



57. Index of refraction, irrespective of form. If a plane-parallel trough, 

 t, containing a solution of mercury potassic iodide, s, is placed between m 

 and m', figure 82 normally to the rays, neither the achromatics nor the 

 spectrum fringes (broad slit admissible) are affected. Inclination to the 

 normal position will change the size of fringes only. Hence if a piece of glass, 

 g, is inserted into the trough with the rays separated as at 2 3 and 8 7, in figure 

 79, one of them only, say 8 7, passing through the glass, the spectrum fringes 

 will change form or vanish, except at that part of the spectrum, in which the 

 index of refraction of the glass and the solution are identical, provided the 

 dispersion coefficients are also the same. It is thus of interest to determine 

 to what degree the method can be practically utilized. 



In the case of bodies of regular form, like lenses, spectrum fringes and even 

 achromatics will usually appear when the sharply seen, fine slit-images coin- 

 cide in the principal focus, i.e., the position of the ocular for parallel rays. 

 But the fringes will as a rule be in other focal planes. When one slit-image 

 is not quite white, and if there is a small angle between them, the fringes 

 will run obliquely through the spectrum from top to bottom. Different 

 regions will be brought out by different focussing of the ocular. By trying 

 out the four edges of the half-silver, one may usually be found in which 

 the vertical coincidence of the slit-images is perfect and these annoyances 

 disappear. 



58. The same. Glass Plate. Lenses. In relation to the principle (57) 

 in question, if a plate of glass of higher refractive index is introduced into the 

 solution, s, figure 82, and transversed by one ray only, the original intensely 

 black, nearly horizontal bands in the spectrum are changed to much finer 

 lines, at a considerable angle (45, etc.) to the horizontal. This inclination 

 is symmetrically down toward the red, or up toward the red, according as 

 one or the other beam traverses the glass. Moreover, the size of the fringes 

 now decreases in much more pronounced ratio from red to violet. The 

 achromatics have necessarily vanished. It would need special compensation 

 (horizontal spectrum fringes) to restore them, and from this compensation 

 the difference of index between solution and glass could be computed. Had 



