F H. Bigelow — Further Study of the Solar Corona. 347 



of the line of force with the tangent, 90 — £ is its angle with 

 the normal. 



If l l is the apparent measured angle of inclination of a line 

 with the radius extended, then tan £=tan I 1 sec a sec d. 



In a preceding paper I applied this theorem to some photo- 

 graphs of coronal lines, finding a strong tendency to represent 

 the intercept ratio in the four quadrants, and yet with such 

 marked exceptions as might well lead to the interpretation 

 that the agreement was fortuitous between the graphical rep- 

 resentations of the mathematical formulae and the natural 

 phenomenon. It is now my object to carry out the comparison 

 with considerable accuracy, paying regard to the distortions of 

 the coronal lines as produced by the perspective effects. It 

 may be possible sometime to include the corrections due to 

 differential refraction, non -coincidence of the centers of the 

 disks of the Sun and the Moon with the axis of vision, and 

 such others as exist, but at present they must be omitted be- 

 cause the photographs used are too small to permit settings for 

 measures within these limits of precision. 



The Negative here discussed is 



Marked No. 16, July 29, 1878, La Junta, Col., by Professor 

 Asaph Hall. It was taken with a Dallmeyer's patent portrait 

 and group lens, size No. 8 D ; effective focal length 37"89 

 inches, clear aperture 6 - inches ; the plates were the dry 

 washed emulsion ; the size of the image of the Moon's disk is 

 0*362 inch, but the limb is not circular because of the Moon's 

 motion in right ascension. The measurements were made by 

 centering the image so that in revolving the table the edge of 

 the disk remained tangent to the micrometer thread. Having 

 selected the polar line of the corona by inspection, the read- 

 ings of the N. and S. poles respectively were taken ; then as 

 many settings on individual rays, as was practicable were made, 

 the polar angle and the distance in micrometer revolutions 

 read. The corresponding values of r and d were made the 

 basis of the computation. 



North Polar Setting 251° 40'. 



South Polar Setting 244° 30'. 



The angle from the N. Pole through the east to the S. Pole is 

 172° 50'. 



The North Pole lies 1° 10' east of Sun's Axis. . 



The South Pole lies 6° 0' east of Sun's Axis. 



The reading for the North Pole is 7*55 divisions. 



The reading for the South Pole is 7*45 divisions. 



Correction for micrometer readings is for N. P. readings —TOO ; 

 for S. P. reading — 0'90. The diameter of the disk is 13-10 

 divisions. The factor for reduction to the unit radius is 6*55. 



A complete record of the readings is given in table No. 1. 



