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Lens, Fresnel. A compound lens formed by placing around a 

 central convex lens rings of prisms so shaped as to have all 

 the same focus ; used in lighthouses. [3, 18, 26.] 



Lens, Photographic. A lens specially designed for taking photo- 

 graphs. [1, 2, 3, 8, 9. 10, 17, 18, 20, 22, 23, 26.] 



Lens, Portrait. A compound photographic lens adapted to 

 portraiture. 



Lens, Rapid Rectilinear. A photographic lens consisting of two 

 symmetrical combinations to avoid distortion. Introduced 

 by J. H. Dallmeyer. 



Lens, Stanhope. A lens with a focus at one of its surfaces. [2, 

 17, 20, 26.] 



Lens, Telephoto. A compound photographic lens giving large 

 pictures at short camera extensions for distant objects. 



Lens, Wide Angle. A photographic lens of short focal length 

 giving a wide angle of view. 



Lens Measurer (Geneva). An instrument for measuring the 

 curvatures of spectacle lenses directly on a graduated dial 

 by the motion of a movable leg between two fixed legs. 



Lenticular Glass. A glass of the form or shape of the seeds of 

 a lentil ; convex. [3.] 



Lessing's Electric Furnace. See Electric Furnace. 



Level. An instrument by which to find a horizontal line or 

 adjust something with reference to a horizontal line, such 

 as surveyor's level, builder's level. [1, 5, 6, 10, 13, 15, 16, 

 17, 18, 19, 20, 21, 24, 25, 26, 27.] 



Level, Abney. Invented by Sir Wm. Abney. A pocket or hand 

 reflecting level consisting of a spirit level, a divided arc and 

 a sighting tube. A reflector in the sighting tube enables the 

 bubble to be seen at the same time as the object. [2, 5, 6, 9, 

 10, 13, 15, 17, 18, 20, 21, 24, 25, 26, 27.] 



Level, Aldis Optical. A form of sensitive level bubble in which 

 the movement of the bubble is greatly magnified by optical 

 means involving the use of the bubble as a negative lens 

 through which a fixed mark is either viewed or has its image 

 projected. 



Level, Builder's. A simple form of the Surveying Level (q.v.). 



Level, Bural. See Reflecting Level below. 



Level, Clinometer. Consists of a small spirit level mounted on an 

 inclinable frame with a divided arc. 



Level, Cushing's. In this pattern the object-glass and the eye- 

 piece can be reversed, thus giving increased accuracy in 

 reading, as in a Y-Level. 



Level, Drainage. Similar to Builder's. 



Level, Dumpy. A surveying level in which the telescope is 

 fixed. [2. 5, 9, 10, 13, 15, 16, 17, 20, 21, 24, 25, 26, 27.] 



Level, Gradient-Telemeter. A special form of surveying level 

 and gradient indicator and telemeter. With this instrument 

 and a levelling staff distances are measured with consider- 

 able accuracy and rapidity. [13, 15, 17, 18, 21, 24, 25, 27.] 



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