A New Ref Tactometer. By Dr. Royston-Pigott. 301 
and this socket carries a small stage accurately turned and ground, 
furnished with stops and a spring to confine the small plate of 
glass, if necessary, in one position. The axis of the screw and of 
the observing-microscope are carefully adjusted in one line, so as to 
have a common collimation. 
In very thin glass an objective of quarter-inch focal length has 
been found sufficient. For thick glass, nearly half an inch thick, 
Mr. Wray, of Highgate, made a beautiful half-inch with three lenses 
cemented together with balsam, so as to give the greatest possible 
penetration. The body of the nucroscope is about six inches long. 
The whole instrument is placed at an angle of about 40° 
upon a mahogany frame. Its accuracy depends on the weight of 
the toothed wheels always bearing with an equal pressure, without 
springs, on the back poppet of the jeweller's lathe employed to 
carry and work the apparatus. 
It remains to say a few words on the method of using the 
instrument. 
1. By transmitted Light. — The instrument is placed near a 
window in daylight, and the small condenser is then used to throw 
a light upon the minute prism, which is then reflected up the mi- 
croscope. A minute drop of kerosine is placed on the prism-lens at 
the end of the micrometer-screw, and then wiped off".* The lens is 
now withdrawn a little below the stage, and the plate of glass to 
be measured is placed upon it. The microscope is armed with a 
quarter-inch, a half-inch, or inch objective, according to the thick- 
ness of the plate to be examined. 
The microscope is then focussed upon the under surface of the 
plate. The micrometer-wheels are set in motion. The prism-lens 
gradually rises in view. The instant of contact is observed by a 
sudden spreading out of the remains of the oil-drop. It may 
require several cleansings or wipings of the lens before the oil is 
sufficiently removed. The film expands and contracts with the 
slightest movement ; with a little practice the eye detects the posi- 
tion of initial contact. 
The instrument is then read. 
Example. — Initial reading for a piece of " covering glass " one 
hundredth of an inch thick : 
I = 0-0044. 
The glass was removed and the prism-lens advanced until its 
surface just came into focus. The distance was then read. 
D = 0-008G 
1 = 0- 0044 initial reading. 
A = 0 • 0042 the elevation of image. 
* Many wipings appear still to leave a minute quantity which the expe- 
riment really requires to be left to form a visible film. 
