76 Lord Rayleigh [March 24, 



arising is to determine the direction of the deviation. This may be 

 effected by observing the displacement of the bands due to a known 

 motion 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 down- 

 wards parallel to itself, so as to increase the thickness of the layer of 

 water, every band w T ould move in a certain direction, 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 downwards, 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 upon 

 the surface, the rule is that the bands, or any parts of them, may be 

 rubbed in the direction of the arrow. 



A good many surfaces have thus been operated upon ; and although 

 a fair amount of success has been attained, further experiment is 

 required in order to determine the best procedure. There is a 

 tendency to leave the marginal parts behind ; so that the bands 

 though straight over the greater part of their length, remain curved 

 at their extremities. In some cases hydrofluoric acid has been, 

 resorted to, but it appears to be rather difficult to control. 



The delicacy of the test is sufficient for every optical purpose. 

 A deviation from straightness amounting to -jV of a band interval 

 could hardly escape the eye, even on simple inspection. This corre- 

 sponds to a departure from flatness of ^o 0I> a wave-length in water, 

 or about ^ of the wave-length in air. Probably a deviation of T ^ A 

 could be made apparent. 



For practical purposes a layer of moderate thickness, adjusted so 

 that the two systems of bands corresponding to the duplicity of the 

 soda line do 'not interfere, is the most suitable. But if we wish to 

 observe bands of high interference, not only must the thickness be 

 increased, but certain precautions become necessary. For instance, 

 the influence of obliquity must be considered. If this element were 

 absolutely constant, it would entail no ill effect. But in consequence 

 of the finite diameter of the pupil of the eye, various obliquities are 

 mixed up together, even if attention be confined to one part of the 

 field. When the thickness of the layer is increased, it becomes 

 necessary to reduce the obliquity to a minimum, and further to diminish 

 the aperture of the eye by the interposition of a suitable slit. The 

 effect of obliquity is shown by the formula 



2 t (1 — cose) = nX. 



The necessary parallelism of the operative surfaces may be obtained, 

 as in the above described apparatus, by the aid of levelling. But a 

 much simpler device may be employed, by which the experimental 



