548 



sciujsrcE. 



[Vol. VI., No. 150. 



times that of Angstrom's map, and in definition 

 are more than equal to any map yet pubhshed, at 

 least to wave-length 5325. The 1474 line is widely 

 double, as also are bs and b4, while E may be rec- 

 ognized as double by the expert. In the region 

 of the H Hne these photographs show even more 

 than Lockyer's map of that region. Negatives 

 have also been prepared down to and including 

 the B grouj), and they may be made ready for 

 pubhcation, one of which shows eleven lines be- 

 tween the D lines. A scale of wave-lengths is 

 printed on each plate, and in no case does the 

 error due to displacement of the scale amount to 

 one part in fifty thousand. The wave-lengths of 

 over 200 lines have been determined to within one 

 part in five hundred thousand, and these serve as 

 reference lines to correct any small error in the 

 adjustment of the scale. The great value of such 

 a map lies not only in the fact that it gives greater 

 detail and is more exact than any other map in 

 existence, but that it actually represents the real 

 appearance of the spectrum in giving the relative 

 intensities and shading of groups of lines so that 

 they are readily recognizable. The photographs 

 were taken with a concave grating six inches in 

 diameter, and having a radius of curvature of 2U 

 feet, and the photographs were taken when the 

 plate was placed directly opposite the grating ; 

 both the sensitive plate and grating being perpen- 

 dicular to a line joining then- centres, and placed 

 at a distance apart equal to the radius of curvature 

 of the grating, the slit being on the circumference 

 of the circle, whose diameter is the distance be- 

 tween the grating and plate. With this arrange- 

 ment, the spectrum is photographed normal for 

 wave-lengths without the intervention of any 

 telescopes or lens systems ; and a suitable scale of 

 equal parts applied to such a photograph at once 

 gives relative wave-lengths. 



F'ew persons have any idea of the jDcrseverance 

 and patience required to bring such a task to a 

 successful issue. More than a year was devoted to 

 preliminary experiments designed to discover the 

 best mode of preparing the plates for the particular 

 regions to be photographed. Hundreds of prepara- 

 tions were tested to find their influence on the sen- 

 sitized plate, and the whole literature of photog- 

 raphy was ransacked, and every method tested to 

 the utmost, before the work of taking the negatives 

 could begin. 



The Rogers-Bond comparator, which has been 

 already referred to as having been purchased by 

 the university lately, is one of two instruments 

 that were constructed in 1881 by Pratt & Whitney 

 of Hartford, Conn. The general plan and require- 

 ments were made out by Prof. W. A. Rogers of 

 Cambridge, and the drawings and details were 



worked out by Mr. George M. Bond, then a student 

 at Stevens institute. The comparator was designed 

 for making exact comparisons of standards of 

 length. The other similar comparator is owned 

 by the Pratt & Whitney manufacturing company, 

 and is used by them in testing and constructing 

 their standard gauges. 



The instrument consists essentially of two micro- 

 scope carriages, which slide on two parallel cylin- 

 drical steel ways between stops, which may be 

 clamped at any point. A carriage entirely in- 

 dependent of the ways on which the microscopes 

 slide, supports the two bars to be compared, and 

 is provided with means of accurate and rapid 

 adjustment, by which the bars maybe successively 

 brought into position under the microscopes, and 

 the lengths compared by the micrometers attached 

 to the microscopes ; or one microscope only need 

 be used, and slid first against the stop at one end, 

 and then against that at the other end. The 

 instrument also affords great facility in determin- 

 ing fractions of a given length with any desired 

 degree of precision. The instrument is one re- 

 quiring the utmost skill in its construction, and it 

 cost several thousand dollars to make it. A full 

 account of this remarkable instrument is given in 

 the Proceedings of the American academy of arts 

 and sciences for 1882-83. K. 



NORTH CAROLINA COAL-FIELDS. 



The coal-deposits of North Carolina have re- 

 cently been examined by Dr. H. M. Chance,^ under 

 the direction of the North Carolina state board of 

 agriculture, with the view of determining their 

 commercial value. 



There are two isolated triassic areas in North 

 Carolina in which coal has been mined, — one on 

 Deep River, and the other on Dan River. Dr. 

 Chance's explorations in the Deep River coal-field 

 consisted mainly in a re-examination of the coal 

 outcrop, which follows the west border of the area, 

 and passes through Farmville, Gulf, and Carbon- 

 ton. The various sections obtained show that in 

 general there are two workable coal-seams in 

 this field, as was proven long ago in the Egypt 

 shaft and at several mines along the coal outcrop. 

 The upper seam averages 2.5 to 3 feet, and the 

 lower 2 feet in thickness. In the Egypt shaft the 

 upper coal measured 4 feet, and the lower 1 foot 

 10 inches; twenty-seven feet below the lowest 

 of these workable seams, another, 1 foot thick, 

 was penetrated. At Gulf three workable seams 

 outcrop, but their thickness is variable owing to 



1 Report on the North Carolina coal-fields to the depart- 

 ment of agriculture \ot North Carolina]. By Dr. H. M. 

 Chance. Raleigh, 1885. 66p., 3 maps. 8°. 



