In order to observe the phenomenon it is necessary to look obliquely on the surface where it appears. But, above all, it is to be remarked that by stooping still more, and looking at the appearance under a more acute angle, the circles not only grow larger but other circles are developed from the centre, of which no trace is to be discovered when we look perpendicularly, even through the strongest magnifiers.
In order to exhibit the phenomenon in its greatest beauty, the utmost attention should be paid to the cleanness of the glasses. If the experiment is made with plate-glass adapted for mirrors, the glass should be handled with gloves. The inner surfaces, which must come in contact with the utmost nicety, may be most conveniently cleaned before the experiment, and the outer surfaces should be kept clean while the pressure is increased.
From what has been said it will be seen that an exact contact of two smooth surfaces is necessary. Polished glasses are best adapted for the purpose. Plates of glass exhibit the most brilliant colours when they fit closely together, and for this reason the phenomenon will increase in beauty if exhibited under an air-pump, by exhausting the air.
The appearance of the coloured rings may be produced in the greatest perfection by placing a convex and concave glass together which have been ground on similar segments of circles. I have never seen the effect more brilliant than with the object-glass of an achromatic telescope, in which the crown-glass and flint-glass were necessarily in the closest contact.