176 0. K. Wead—Millimeter Screw. 
W.L. Makers. Authority. Remarks. 
530 Fraunhofer. Vogel. 
465 Schroder, . 
494 Grubb. os 
490 Clark. Young. Princeton telescope. 
522 ie eo Dartmouth. 
495 > Hastings. Edgecomb’s. 
497 : es Wesleyan Univ. 
duced by a small crown glass prism, is W. L. 5005. Hence, 
in practice, determine the indices of refraction for the lines ©, 
W. L. 561°4 and W. L. 500% in both materials to be used; 
calculate the curves of the objective so that the spherical aber- 
ration for ray 561-4 shall vanish while marginal rays C an 
500% havea common focal point. This method, though simple 
and direct, is without doubt sufficiently correct for all telescopes 
of moderate apertures and all varieties of glass hitherto used in 
their construction. 
Johns Hopkins University, Jan., 1882. 
pean 
Art. XV.—To Cut a Millimeter Screw ; by Cuarztes K, WEAD. 
