298 Transactions of the Royal Microscopical Society. 
and consequently 
t 
M = -• . 
V 
Upon constructing an instrument roughly the index was 
readily found to two places of decimals in covering glass, viz. 
The contrivance of an instrument to read to the 100,000th of 
an inch now seemed desirable. It was necessary not only to mea- 
sure 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 ofi' 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 developing the rings 
suitable in size for the microscopic field of view. 
For being apprised of the near approach of contact, still greater 
convenience was accorded by using a minute film of paraffin oil, 
formed by repeatedly wiping the surface of the screw-lens. Contact 
was then heralded instantly between the two surfaces by a brilliant 
flash of colours. One thing appeared certain, the various colours 
could be produced in perpetually expanding and vanishing rings, 
always starting from the centre. ' I counted no less than thirty- two 
