a distance equal to two-thirds of the numerical value of the 



focal length of either. [2, 3, 8, 10, 11, 13, 17, 18, 19, 20, 21, 



22, 23, 24, 25, 26, 27.] 

 Eye-Protectors. A special form of spectacles with coloured or 



tinted glasses for protecting the eyes from glare, dust, etc. 



See Goggles. [2, 9, 13, 15, 18, 20, 21, 26, 28.] 

 Eyeshade. A hood or shield on an eyepiece to cut off light 



from the employed or unemployed eye. [2, 3, 8, 18, 20, 21, 



22, 26.] 

 Eye-Speculum. A curved reflector with a small central aperture 



used by ophthalmic surgeons for concentrating light on to 



the eye. [20.] 



Fabry and Perot Interferometer. See Interferometer. 



Facila Set-Squares. For the use of draughtsmen. [6, 13, 15, 

 20, 21, 24, 27.] 



Faciometer. An instrument designed to measure the face for 

 spectacle -fitting. [20.] 



Fahrenheit's Hydrometer. See Hydrometer. 



Fairy Sinker. See Sinker. [13, 28.] 



Faraday Prism. See Prism. [13.] 



Fannar's Profit-Calculating Rule. A special form of slide-rule 

 (q.v.}. [5,13,15.] 



Fearnley's Universal Calculator. See Calculator. 



Feed-Water Thermometer. See Thermometer. [5, 13, 15, 16, 

 20, 21, 28.] 



F6ry Radiation Pyrometer. See Pyrometer. [4, 13, 21.] 



F6ry Spiral Pyrometer. See Pyrometer. [4, 13, 21.] 



Field and Oust Tide Recorder. See Tide Recorder. [4, 13, 21.] 



Field Glass. A binocular telescope in compact form for hand 

 use and having somewhat higher powers than an opera -glass. 

 See also Binocular. [2, 3, 5, 9, 10, 13, 15, 16, 17, 18, 20, 21, 

 24, 25, 26, 27, 28.] 



Field -Plotter.= An instrument used for mechanically solving 

 triangles in gunnery. [13, 18, 20, 23, 27.] 



Fighting Map Board. A transparent celluloid sector and 50 arc, 

 with a range-scale, a board of 6-ply wood with two brass 

 arms, and a waterproof case. [20.] 



Filar Micrometer. See Micrometer. [2, 3, 17, 18, 20, 22, 27.] 



Filter (Light). A coloured plate or layer of fluid for filtering 

 light by allowing only light of definite wave-length to pass. 

 [2, 20.] 



Finder. As applied to the telescope, a small low-power tele- 

 scope fitted to the side of the large one for finding the 

 object. As applied to the microscope, a means for regis- 

 tering the position of an object on the slide so that it can 

 readily be found again. Also an auxiliary optical apparatus 

 fitted to a photographic camera to enable the object to be 

 viewed (Viewfinder). (2, 3, 8, 9, 13, 17, 18, 20, 21, 27.] 



