APPENDIX. 917 



Fresnel's wave surface (3) : 



The lines are in the direction of the vibrations of the two polarized 



rays. 



Steiner's surface (2). 

 Twisted cubics (2) : 



{x a sin 20 (vertical). 

 y = b sin 30 (horizontal). 

 z =. c cos 50 (perpendicular to paper). 

 {x = a sin 29. 

 y = b sin 30. 

 z = c sin 70. 



Icosehedron in octahedron. 

 22 in all. 



166b. Heal Image Stereoscope for showing the above. 



Prof. Clerk Maxwell. 



The observer places his eyes about two feet from the large lens, and sees 

 the united real images of the figures at or near the surface of the lens. 



217a. Whitworth's Fluid Compressed Steel. 



Sir Joseph Whitworth fy Co., Limited. 



This hoop was cast from a " Siemens' " furnace and compressed while in 

 its fluid state. This metal when forged is used for Whitworths' guns, screw 

 propeller shafts, linings for steam engine cylinders, crank shafts, &c. &c. 



217b. Vertical Hydraulic Testing Machine. 



Sir Joseph Whitworth fy Co., Limited. 



This machine is used for testing the tensile strength of metals ; the 

 specimens are made cylindrical, the central part being two inches long, and 

 having a cross sectional area of 5 of a square inch. 



223a. Model of an Ancient Egyptian Standard Cubit, 



dated in the reign of King Horns, 9th Pharaoh of the 18th dynasty 

 (1657 B.C.). Mrs. Chisholm. 



The ancient standard measure, of which this is a copy, was found in the 

 ruins of Memphis, and is now in the Royal Museum at Turin. It is a Royal 

 cubit of seven palms or 28 digits. The total length of this end standard 

 measure is 523 '5 millimetres or 20*6 inches, and agrees very nearly with that 

 of several other ancient Pharaonic cubits still existing, as well as with the 

 length of the Royal cubit as deduced by Sir Isaac Newton from the dimensions 

 of the Great Pyramid, the mean length being 525 millimetres. The original 

 natural cubit, or cubit of a man, of 6 palms is also marked upon this measure, 

 being equal to 463 millimetres or 18 '24 inches, and also the ancient Egyptian 

 foot of 16 digits, or of the natural cubit, and equal to 12' 16 inches, or 1-013 

 English foot. The great span of 14 digits and the small span of 12 digits are 

 also marked. 



24Oa. Micrometrical Divisions in English and Metric 

 Measure. Dumoulin Froment, Paris. 



246 a. Wollaston's Single Lens Micrometer. 



Wollaston Collection, Cavendish Laboratory, Cambridge. 

 (Phil. Trans., 1813, p. 119.) 



