



CHAPTER I. 



THE OPEN MERCURY MANOMETER READ BY DISPLACEMENT 



INTERFEROMETRY. 



1. Apparatus. This is practically a U-tube, AmA 1 ', figure i, with wide 

 shanks A, A' connected by a channel m below. A and A' are cylindrical 

 hollows, 2 to 3 cm. deep and about 5 cm. in diameter, cut in a rectangular 

 block BB f , preferably of iron. The connection m must also often be large in 

 section, so as to admit of rapid flow from A to A'. The U-tube is charged with 

 mercury, MmM', M and M' being as shallow as possible to counteract the 

 tendency to vibration. Thin plane parallel glass plates, gg' ', round disks of 

 equal thickness and diameter, are floated on the mercury, which act as mir- 

 rors for the interferometer beams L' and L" and also materially check the 

 tendency of the pool of mercury to vibrate. It would be desirable to be able 

 to use the mercury surfaces at M and M' directly without the intervention of 

 the plate, but all attempts to do this, within the city limits, were at first fail- 

 ures. Moreoever, the amplitude of vibration in the trough further from the 

 source of light is much in excess. Later I found the fringes, but could not use 

 them with advantage. 



The top of the iron block BB' is recessed as shown, to receive the plane par- 

 allel glass plates G, G 1 . These, like g, g', must be equally thick, otherwise the 

 fringes will be multiplied and faint. The annular space cccc between G and B 

 is filled with resinous cement, poured in the molten state. The air-space AA', 

 shut off in this way, communicates with the atmosphere by two tubulures, 

 t and f', in the front side. 



The ray parallelogram of the quadratic interferometer, of which L', L" are 

 the interfering rays, should be vertical. The displacement of the achromatic 

 fringes of white light are read off by a telescope with an ocular micrometer 

 (scale-part o.oi cm.). The fringes parallel to the divisions of the micrometer 

 are conveniently made a scale part in size. The block BB' should be 

 mounted separately from the interferometer. If it is placed on the base of the 

 latter, all manipulations there shake the mercury in BB' and it is necessary 

 to wait for subsidence. This, however, occurs very soon, so the separate 

 mounting is not absolutely necessary. Without manual interference the 

 fringes are about as quiet as in a solid apparatus. 



2. Experiments. To test this apparatus the air-space A A 1 was left with 

 a plenum of air. With A' communicating with the atmosphere, A was joined 

 through / and a filamentary capillary glass or metal tube to an apparatus by 

 which slight pressure could be applied. In the first trials I attempted to use a 

 water manometer controlled by a micrometer-screw ; but the vibrations of the 

 meniscus were at once impressed on MM', so that the fringes were hard to 

 keep at rest. I then devised the apparatus shown in figure 2, which is merely 



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