Prism on Newton's Rings. 349 
obstruction usually encountered in trying to bring two 
surfaces nearly enough together to exhibit the rings o£ low 
order. In favour of the view that the obstacle is merely 
dust and fibres, I remember instancing the ease with which 
a photographic print, enameled by being allowed to dry in 
contact with a suitably prepared glass plate, could be brought 
back into optical contact after partial separation therefrom. 
My recent observations with the glass plates point entirely 
in the same direction. However carefully the surfaces are 
cleaned by washing and wiping — finally with a dry hand, 
the rings of low order can usually be attained only at 
certain parts of the surface *. If we attempt to shift them 
to another place chosen at random, they usually pass into 
rings of higher order or disappear altogether. On the other 
hand, when rings of low order have once been seen at a 
particular place, it is usually possible to lift the upper glass 
carefully and to replace it without losing the rings at the 
place in question. I have repeatedly lifted the glass when 
the centre of the system was showing the white of the first 
order or even the darkening (I do not say black) corre- 
sponding to a still closer approximation, and found the colour 
recovered under no greater force than the weight of the 
glass. Some time is required, doubtless in order that the 
air may escape, for the complete recovery of the original 
closeness ; but in the absence of foreign matter it appears 
that there is no other obstacle to an approximation of 
say \\. 
In making the observations it is convenient to introduce 
a not too small magnifying lens of perhaps 8 inches focus 
and to throw an image of the source of light upon the pupil 
of the eye. With the glasses in contact it is easy to trace 
the rise in the order of the achromatic band as the eye 
and prism are drawn back. As regards the latter a direct- 
vision instrument of moderate power (three prisms in all) is 
the most suitable. An interval between the glasses may be 
introduced by stages. When the approximation is such as 
to show colours of the 3rd or 4th orders at the centre, it 
becomes apparent that the best achromatic effects are attained 
when the prism is at a certain distance, and that when this 
distance is exceeded the more achromatic places are separated 
* The plates are here supposed to be brought together without sliding. 
By a careful sliding together of two surfaces, the foreign matter may be 
extruded, as in Hilger's echelon gratings, where optical contact is 
attained over considerable areas. 
Phil. Mag. S. 6. Vol. 15. No. 87. March 1908. 2 B 
