June 29, 1893] 



NATURE 



213 



considerable — bands of high interference as they are called — 

 the most natural course is to employ approximately homo- 

 geneous light, such as that afforded by a soda flame. Unfortu- 

 nately, this light is hardly bright enough for projection upon a 

 large scale. 



-A partial escape from this difficulty is afforded by Newton's 

 observations as to what occurs when a ring system is regarded 

 through a prism. In this case the bands upon one side may 

 become approximately achromatic, and are thus visible to a 

 tolerably high order, in spite of the whiteness of the light. 

 Under these circumstances there is, of course, no difficulty in 

 obtaining sufficient illumination ; and binds formed in this way 

 were projected upon the screen.' 



The bands seen when li^ht from a soda flame falls upon 

 nearly parallel surfaces have often been employed as a test of 

 flatness. Two flat surfaces can be made to fit, and then the 

 bands are few and broad, if not entirely absent ; and, however 

 the surfaces may be presented to one another, the bands should 

 be straight, parallel, and equi-distant. If this condition be 

 violated, one or other of the surfaces deviates from flatnes*. In 

 Fig. 2, A and B represent the glasses to be tested, and C is a 

 lens of two or three feet focal length. Rays diverging from a 

 sorta flame at E are rendered parallel by the lens, and after 

 reflection from the surfaces are re-combined by the lens at E. 

 To make an observation, the coincidence of the radiant point 

 an! its imxge must be somewhat disturbed, the one being dis- 

 placed to a position a little beyond, and the other to a position 

 I a little in front of, the diagram. 



The eye, protected from the flime by a suitable screen, is 

 placed at the image, and being focused upon AB, sees the field 



plates were seen grooves due to rubbing with rouge along 

 defined track, and depressions, some of considerable regularity, 

 obtained by the action of diluted hydrofluoric acid, which was 

 allowed to stand for some minutes as a drop upon the surface 

 of the glass. 



By this method it is easy to compare one flat with another, 

 and thus, if the first be known to be free from error, to de- 

 termine the errors of the second. But how are we to obtain 

 and verify a standard? The plan usually followed is to bring 

 three surfaces into comparison. The fact that two surfaces can 

 be made to fit another in all azimuths proves that they are 

 spherical and of equal curvatures, but one convex and the other 

 concave, the case of perfect flatness not being excluded. If A 

 and B fit another, and also A and C, it follows that B and C 

 must be similar. Hence, if B and C also fit one 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 com- 

 plete, its application in practice is extremely tedious, especially 

 when the surfaces are not of revalution. A very simple solu- 

 tion 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 desiiable to allow a margin of about I J inch. 

 The surface to be tested is supported horizontally at a short 

 distance (xV or ^V 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 approximate parallel- 



FlG. 2. 



traversed by bands. The reflector D is introduced as a matter 

 of convenience to make the line of vision horizontal. 



These bands may be photographed. The lens of the camera 

 lakes the place of the eye, and should be as close to the flame 



possible. With suitable plates, sensitised by cyanin, the ex- 

 posure required may vary from ten minutes to an hour. To get 

 the best results, the hinder surface of A should be blackened, 

 and the front surface of B should be thrown out of action by the 

 superposition of a wedge-shaped plate of glass, the intervening 

 space being filled with oil of turpentine or other fluid having 

 oeatly the same refraction as glass. Moreover, the light should 

 be purified from blue rays by a trough containing solution of 

 bichromate of potash. With these precautions the dark parts of 

 ihe bands are very black, and the exposure may be prolonged 

 ranch beyond what would otherwise be admissible. 



The lantern slides exhibited showed the elliptical rings indi- 



ative of a curvature of the same sign in both directions, the 



llyperbohc bands corresponding to a saddle-shaped surface, and 



'he approximately parallel system due to the juxtaposition of 



vo telescopic " flats," kindly lent by Mr. Common. On other 



,., !,''°'''=°''y.'s given in .a paper upon "Achroma-ic Interfersnca Bands," 

 ' Int. Mag., August 1889.' 



Fig. 3. 



ism with the water. In practice the principal trouble is in the 

 avoidance o tremors and motes. When the apparatus is set up 

 on the floor of a cellar in the country, the tremors are suffici- 

 ently 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 aimost 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 surprisingly 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 bands. Fig. 3 represents hands 

 actually observed from the face of a prism. That these are not 

 straight, parallel, and equi-distant is a proof that the surface 

 deviates from flatness. The question next arising is to determine 

 the direction of the deviation. This may be effected by ob- 

 serving the displacement of the bands due to a known molion 

 of the levelling screws ; but a simpler process is open to us. 

 It is evident that if the surface under test were to be moved 

 downwards parallel to itself, so as to increase the thickness of 

 the layer of water, every band would move in a certain direc- 

 tion, viz. towards the side where the layer is thinnest. What 

 amounts to the .same, the retardation may be increased, without 

 touching the apparatus, by so moving the eye as to diminish 

 the obliquity of the reflection. Suppose, for example, in Fig. 3 

 that the movement in question causes the bands to travel down- 

 wards, as indicated by the arrow. The inference is that the 

 surface is concave. More glass must be removed at the ends 

 of the bands than in the middle in order to straighten them. If 

 the object be to correct the errors by local polishing operations 

 ' 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 



10^. 



NO. 12 



oOi 



VOL. 



48] 



