A New Bef Tactometer. By Dr. Boyston-Pigott. 
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sistently adhering to the surface of the lens, in spite of repeated 
wipings, is worthy of notice. 
On the near approach of the lens to the under surface of the glass 
under notice, the scattered drops suddenly coalesce, shooting out 
into a film of varying colour. 
On one occasion a small oil-drop, one hundredth of an inch in 
diameter, appeared as a black annulus enclosing a bright thin ring 
of light, which enlarged on being touched by the prism-lens by the 
advance of the screw. It spontaneously then spread out and rapidly 
exhibited within its centre a sudden display of very minute but 
richly coloured Newtonian rings, formed in this case by interior 
reflected light, although transmitted light was then being employed. 
It is not absolutely necessary that a plate of glass with pre- 
cisely parallel sides be used. A wedge can be manipulated if a 
particular spot be chosen and the wedge be most carefully adjusted 
to the same position by means of the stop and ledge on the stage. 
Less difficult, however, is glass formed into a plano-convex lens of 
long focus, the plane side being placed downwards, and the same 
point, the summit, if possible, being always selected for observation ; 
better still if a shce be cut off so as to present a secure fixing of the 
lens in the same position. 
A variety of substances formed into plates, wedges, or lenses, 
with little convexity, may thus be examined, as also fluids enclosed 
between parallel plates, as well as enclosed in a tube, as already 
described ; or by the instrument exhibited under the condition that 
the fluid be enclosed in a thin glass cell, which is completely to be 
filled with the given fluid. 
The instrument is by no means intended to compete with the 
spectroscope. But it gives the mean refraction in a variety of 
cases where that instrument is not applicable. 
