1893.] on Interference Bands and their Applications. 



75 



another, all three surfaces must be flat. By an extension of this 

 process the errors of three surfaces which are not flat can be found 

 from a consideration of the interference bands which they present 

 when combined in three pairs. 



But although the method just referred to is theoretically complete, 

 its application in practice is extremely tedious, especially when 

 the surfaces are not of revolution. A very simple solution of the 

 difficulty has been found in the use of a free surface of water, which, 

 when protected from tremors and motes, is as flat as can be desired.* 

 In order to avoid all trace of capillary curvature it is desirable to 

 allow a margin of about 1J inch. The surface to be tested is sup- 

 ported horizontally at a short distance (y 1 ^ or -^ inch) below that of 

 the water, and the whole is carried upon a large and massive levelling 

 stand. By the aid of screws the glass surface is brought into approxi- 

 mate parallelism with the water. In practice the principal trouble 

 is in the avoidance of tremors and motes. When the apparatus is set 

 up on the floor of a cellar in the country, the tremors are sufficiently 

 excluded, but care must be taken to protect the surface from the 

 slightest draught. To this end the space over the water must be 

 enclosed almost air-tight. In towns, during the hours of traffic, it 

 would probably require great precaution to avoid the disturbing 

 effects of tremors. In this respect it is advantageous to diminish the 

 thickness of the layer of water ; but if the thinning be carried too 

 far, the subsidence of the water surface to equilibrium becomes sur- 



Fig. 3. 



prisingly slow, and a doubt may be felt whether after all there may 

 not remain some deviation from flatness due to irregularities of 

 temperature. 



With the aid of the levelling screws the bands may be made as broad 

 as the nature of the surface admits ; but it is usually better so to adjust 

 the level that the field is traversed by five or six approximately parallel 

 bauds. Fig. 3 represents bands actually observed from the face of a 

 prism. That these are not straight, parallel, and equidistant is a 

 proof that the surface deviates from flatness. The question next 



* The diameter would need to be 4 feet in order that the depression at the 

 circumference, due to the general curvature of the earth, should amount to -^ A. 



