BRIEF HISTORICAL SUMMARY 687 



Thompson, p. 953-954) . While this works well for general lighting, it is shown 

 in the preceding pages (553-566) that alternating current is far inferior to 

 direct current for projection purposes. 



At first the carbons were both horizontal (fig. 413), then they were made 

 vertical, and later at various angles of inclination. In order to keep the crater 

 of the positive carbon constantly in the optic axis, Mr. Albert T. Thompson of 

 Boston manufactured and used, especially for projection purposes, an arc lamp 

 in which the carbons are at right angles, the positive carbon being horizontal 

 and hence constantly in line with the axis of the projection instrument. This 

 was in 1894.* From that time onward the advantage of this position has 

 become more and more appreciated, and the superiority for projection purposes 

 is shown graphically in the curve given in Chapter XIII (fig. 302). 



The following is the statement of Mr. Albert T. Thompson concerning the 

 90 arrangement of the carbons in an arc lamp for the magic lantern: 



BOSTON, Dec. 6, 1907. 



"Replying to your valued communication of the 2d, I will state that I first 

 manufactured the 90 arc lamps in 1894 an d a careful search of all arc lamp 

 and stereopticon catalogs published about that period, fails to show arc lamps 

 of the 90 construction. 



"I did not patent the lamp, for at that time there was no demand for them, 

 and of course it was difficult to look into the future and realize that in a few 

 years thousands and thousands would be sold. 



"The facts to the best of my knowledge and belief were never published in 

 any scientific journal. 



Yours very truly, 



A. T. THOMPSON." 



