1876.] a new Ref Tactometer, 397 



position. The axis of the screw and of the observiug-microscope are 

 carefully adjusted in one line, so as to have a common collimation. 



In very thin glass an objective of 5-inch focal length has been found 

 sufficient. For thick glass, nearly half an inch thick, Mr. Wray, of High- 

 gate, made a beautiful half -inch with three lenses cemented together with 

 balsam, so as to give the greatest possible penetration. The body of the 

 microscope is about 6 inches long. 



The whole instrument is placed at an angle of about forty degrees. 

 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 microscope. 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 The lens is then withdrawn a little below 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 thickness of the plate to be examined. 



The microscope is then focused upon the under surface of the plate. 

 The micrometer-wheels are set in motion. The prism-lens gradually 

 rises into 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 position of initial contact. 



The instrument is then read. 



Example. — Initial reading for a piece of "covering-glass" one hun- 

 dredth of an inch thick, 



1=0-0044. 



The glass was removed and the prism-lens advancad until its surface 

 just came into focus. The distance was then read. 



D = 0-0086 



I = 0-0044 initial reading. 



A = 0*0042 the elevation of image. 



Replacing the glass and again repeating the process the initial reading 

 at the lower contact was now found not to be 0*0044, but 



I 2 = 0-0045 (an extra yjnrWu" i ncn )« 



Viewing an exceedingly minute scratch on the upper surface by 

 refocusing upon it, the reading for thickness was 



