96 



THE INTERFEROMETRY OF 



the mirrors. The rays in such a case are all very nearly parallel. Sunlight, 

 arc light, and the Nernst filament were each available for illumination. If 

 the latter is used, the adjustment must be made by aid of the two white slit 

 images, which are to coincide horizontally and vertically. Otherwise the 

 sodium lines are available. With a very long collimator (2 meters) and a 

 wide single-lens objective (10 cm. or more), the Nernst filament may be used 

 directly in place of the slit. If the beam passing the objective is not wider 

 than i cm. (opaque slotted screen), very perfect ellipses may be obtained. 

 On inserting the trough with a thickness of 1.3 cm. of CS 2 solution normally 

 into the MH beam, the original very large ellipses were reduced in size and 

 rounded as usual to smaller circles. Submerging a convex lens (i diopter) 

 into the liquid until the beam passed symmetrically through it changed 

 these circles to very long horizontal spindles. A concave lens similarly pro- 

 duced horizontally very eccentric hyperbolae. With water in the trough, only 

 the convex lens showed observable fringes, these being very long, practically 

 linear horizontal spindles. All these fringes lie considerably in front of the 

 principal focal plane of the telescope (fig. 54, T), and the abnormal forms are 

 necessarily relatively faint. They change in shape and intensity with the 

 focal plane observed. 



63 



On mixing CS 2 with kerosene (about equal parts), types of fringes shown 

 in figure 63, but with many more lines, were obtained. This is a combina- 

 tion of both spindles and hyperbolae. Probably three layers of liquid are 

 chiefly in question, viz, kerosene, kerosene + CS 2 , CS 2 , and the three stages 

 of form and the sinuous lines correspond to them. Fringes were sharp only if 

 viewed in front of the principal focal plane of the telescope. By submerging 

 convex or concave lenses, the hyperbolic parts or the spindles of the fringes 

 could often be removed. Similar results were obtained with mixture of CSz 

 and sweet oil, though this solution is more homogeneous. 



50. Apparatus. To obviate the tremor of apparatus which is inevitable in 

 the case of the long-distance interferometer, the parts were now screwed 

 down at short distances in the cast-iron block B, figure 64. Here the ranges 

 MH, HN of half -silvered plate H, and opaque mirrors M, N, did not exceed 



