394 Dr. G. W. Royston-Pigott on [Mar. 23, 



A thin glass cover about one hundredth of an inch thick gave 



ju=l-5502. 

 The optical equation for a plate of glass, 



i t 



(where u is the distance of the object and v the conjugate focus), points 

 out that when the object is on the surface, or w=0, 



t 



and consequently 



t 



Upon constructing an instrument roughly the index was readily found 

 to two places of decimals in covering- glass, viz. ^ = 1-55. 



The contrivance of an instrument to read to the 100,000th of an inch 

 now seemed desirable. It was necessary not only to measure thickness, 

 but the elevation of the image within the substance of the plate. 



The well-known delicacy of evanishment of a point under a good 

 microscope seemed to afford an exquisite test of distance. 



At first the point of an advancing screw was made to touch the 

 " plate," the screw moving in the axis of the microscope. The point 

 was brought into focus, or rather its image. The refracting plate was 

 now removed, the microscope remaining undisturbed. The screw was 

 then advanced until its point again came into focus ; the point now 

 occupied the precise position just before occupied by its image. This 

 distance was then read off and the thickness of the plate read by 

 observing successively the distances traversed by the screw between the 

 point of first contact and its rising to the very same position of a 

 particle already observed on the upper surface. 



II. At this point of the research it occurred to the writer to substitute a 

 minute plano-convex lens, fixed to the end of the screw, and endeavour 

 to produce contact-films, especially the black central spot of Newton's 

 rings. 



In order to render the colours of these rings gorgeous by reflected 

 light, a thin piece of plate glass was fixed at an angle of 45° behind the 

 observing objective, and a hole perforated in the tube or " body " to 

 admit light from a lamp ; the object-glass then condensed a strong light 

 upon the film-forming surfaces, and the black central spot came out 

 beautifully black and distinctly defined in the field of the microscope. 

 This occurred whether air or other fluid intervened between the lens and 

 the surface. After fixing several different lenses, I found a radius of 

 curvature of about one quarter of an inch the most convenient for deve- 

 loping the rings suitably for the microscopic field of view. 



Tor being apprised of the near approach of contact, still greater con- 



