330 Mr. S. Skinner on the Occurrence 
If when the liquid has been introduced the upper lens be 
rolled on the lower the observer sees following the central 
dark spot, viewed by reflected light, a crescent-shaped space, 
very bright provided the illumination be sufficiently oblique. 
This is a vacuous or vapour-filled space, for when the motion 
of rolling ceases the liquid flows into the space and com- 
pletely fills it. Star ting the rolling in the opposite direction 
the same appearances are seen In the reverse order. It was 
the observation of this space which led me to make the 
following experiments, and at the time I was unaware that 
Newton had observed its formation, for the paragraph in 
which he mentions it is omitted from the quotations which 
have found a place in modern text-books. In Observation 11 
Newton writes:—‘‘ When the Water was between the Glasses, 
if I pressed the upper Glass variously at its edges to make 
the Rings move nimbly from one place to another, a little 
white Spot would immediately follow the center of them, 
which upon creeping in of the ambient Water into that place 
would presently vanish. Its appearance was such as inter- 
jacent Air would have caused, and it exhibited the same 
Colours. But it was not Air, for where any bubbles of Air 
were in the Water they would not vanish. The Reflexion 
must have been caused by a subtiler Medium, which could 
recede through the Glasses at the creeping in of the 
Water.” 
§ 2. The inflow of the liquid to. fill the vacuous space 
must depend in some way on its viscosity. When a more 
viscous liquid is used all the effects are more pronounced, 
and with glycerine or lubricating oil quite large vacuous 
spaces, frequently broken up into a number of small bubbles, 
are obtained. The method of observation with oblique light 
takes advantage of total internal reflexion, and consequently 
necessitates a certain angle, depending on the refractive 
index of the glass, and it may be inconvenient. Another 
mode of observing the space is to use sodium light, when the 
relatively bright ‘Newtonian rings in the space “show up well 
in contrast with the faint rings in the liquid. 
§ 3. A third mode of observation, which is much the most 
convenient, is to use a deeply coloured liquid and to look at 
the space by transmitted light. I have found that a very 
convenient liquid is a strong solution of fuchsin in glycerine. 
This red solution is so deeply coloured that even up to the 
point where the lenses are nearest some colour shows, and 
that light is only brightly transmitted at the place where 
there is a break in the liquid. Placing this liquid between 
the lenses, and using daylight reflected from a sheet of white 
