312 Intelligence and Miscellaneous Articles. 



upon a plane surface of glass and the system is illuminated by an 

 obliquely incident beam of monochromatic light, as, for example, by 

 a sodium-flame, the well-known phenomenon of Newton's rings is 

 observed with remarkable distinctness and perfection of definition. 

 The symmetry of the rings will depend, in part, on the perfection of 

 figure of the lens, in part on that of the plane surface. An extremely 

 minute deviation from a perfect plane will produce a marked distor- 

 tion of the circular figure of the ring nearest the centre. That this 

 distortion is or is not due to the lens may be determined by rotating 

 the lens round its optical axis normal to the plane. No change of 

 figure will be seen if the lens is perfect in form and the inequality is 

 in the plane surface only. Different parts of the plane surface may 

 of course be tested in succession, by moving the lens from point to 

 point ; and, if necessary, the rings may be observed with a telescope. 



Prof. Eood, of New York, has suggested for the observation of 

 Newton's rings a method which permits of the employment of a lens 

 of comparatively small radius of curvature and a microscope. In his 

 arrangement the lens and plate of glass are placed upon the stage of 

 the microscope, the light from beneath being cut off ; and monochro- 

 matic light is then thrown down upon the system by means of a 

 plate of glass with parallel surfaces inclined to the axis of the micro- 

 scope at a convenient angle, and placed between the objective and 

 the plano-convex lens. In this manner the rings are seen with great 

 distinctness and beauty, and the arrangement is particularly compact 

 and convenient. 



The interference bands of Talbot afford a method not merely of 

 observing with great precision the inequalities of surface and want 

 of parallelism of the faces of plates of glass, but also of photographing 

 these defects and obtaining a permanent chart of the glass which may 

 be of material assistance in correcting its figure. It is only neces- 

 sary for this purpose to place the glass to be examined near to the 

 object-glass of the collimator and perpendicular to its axis, so as to 

 intercept that half of the bundle of parallel rays which falls upon the 

 first surface of the first prism nearest its refracting edge. If the 

 plate has perfectly plane and parallel surfaces, the interference bands 

 will be sharply defined and parallel in the whole field of view. The 

 slightest inequality of surface or inclination of the faces will pro- 

 duce curvature or distortion of the bands ; and if the eyepiece of the 

 observing-telescope be removed, the image maybe received on a sen- 

 sitive plate and photographed. The number of prisms to be employed 

 in a particular case will depend upon the thickness of the plate of 

 glass examined, and, in general terms, upon its dispersive power. 

 For a piece of French plate glass four millimetres in thickness, two 

 bisulphide-of-carbon prisms of 60° must be used to produce a suffi- 

 cient separation of the interference bands to enable them to be seen 

 distinctly. More prisms must be used for thicker plates ; and in this 

 way a limit is soon reached at which the method ceases to be appli- 

 cable. — Silliman's American Journal, July 1870. 



