G. II. Warren — A Graduated Sphere. 493 



Art. L. — A Graduated Sphere for the Solution of Problems 

 in Crystal Optics; by Charles H. Warren. 



Among the many useful adjuncts to work in crystal optics 

 is the graduated, porcelain hemisphere devised by Nikitin.* 

 Inasmuch as it has been impossible, since the outbreak of the 

 European war, to procure this piece of apparatus, it occurred 

 to the writer to attempt the construction of a piece of apparatus 

 to take its place. The result was so successful that it has 

 seemed worth while to publish a brief description of this new 

 sphere, with the idea, that others engaged in work in crystal 

 optics, and desiring such a piece of apparatus, might find the 

 description useful in constructing a similar one. 



While the general design of the sphere was the writer's, the 

 greatest credit is due to Mr. Carl Selig, mechanician for the 

 Department of Physics at the Massachusetts Institute of 

 Technology, for his skill and ingenuity in working out the 

 details of the construction. 



The material first used for the sphere was an eight-inch 

 bowling-alley ball. This was found, however, to depart 

 slightly from a true sphere, so that a hollow brass sphere was 

 substituted. This was cast with a shell of about -if" thickness, 

 and the surface was then machined down on a lathe to a per- 

 fectly spherical shape. 



The sphere was next given three coats of white enamel 

 paint, and then polished with pumice and water. Vertical and 

 horizontal meridians, ten degrees apart, were then ruled on the 

 enamel surface, this being done on a lathe, using black drawing 

 ink. 



The graduated sphere was then mounted in a hollow metal 

 cup about five inches in diameter, which was carefully lined 

 with felt (see a, fig. 1). The cup was accurately centered 

 with reference to the holding frame with its attached scales, 

 and so mounted on a post as to allow the cup and sphere to be 

 rotated rigidly in the horizontal plane. Three short, brass 

 pins (see fig.) serve as handles to make the rotation easier. 



The standard carrying the sphere is surmounted by three 

 polished steel scales mounted on a brass backing \" thick. 

 One is a horizontal scale (b, fig.) and two are vertical scales, 

 ninety degrees apart. The three scales are graduated in 

 degrees, every five-degree mark being accentuated, and every 

 ten-degree graduation being numbered. One of the vertical 

 scales (c, fig.) is mounted so that it can be rotated about a 

 horizontal diameter through a range of 125°. To make such 



*Zeitschr. f. Kryst., xlvii, 381, 1910. 



