BLUE-VIOLET LIGHT IN THE SOLAR CORONA ON AUGUST 30, 1905. 313 



negatives. Tli- miM^im-nu-iits tiiinMu-<l 7'. DOTW f e,|ii.il il.-iisily. \\liirli !!,, n^ t<> 

 mean distances between 0'12 and 1 solar diameter. As each of the two photographs 

 has a background of equal density all round, these curves will be employed in reducing 

 measured distances on other photographs as described in 4 (c) and 4 (d). 



The apparatus which I employed in measuring the photographs consists of a low- 

 power microscope mounted on a slide whose position can be read by vernier to O'OOl 

 of an inch. The slide is mounted on a circular plate which turns in a ring, so that 

 measurements can be made at any position -angle. 



The measurements were made at position -angles degree, 15 degrees, <fec. The 

 position-angles refer to the north pole of the sun. I obtained the zero of the position- 

 angles from the calculated position-angles referred to the north pole of the sun and 

 the positions of the second and third contacts on the first and last photographs. The 

 position-angles of the contacts referred to the celestial pole were found from the data 

 given in the 'Nautical Almanac' (124 degrees and 287 degrees), and the position- 

 angle of the north pole of the sun is 207 degrees. 



On the photographs published with this paper the line joining the centres of the 

 lunar discs has a position-angle of 239 degrees, 59 degrees being to the left and 

 149 degrees at the top. 



1 observed the following rule in measuring : After clamping the microscope at a 

 certain position-angle, 1 set the wire successively on the moon's limb, then on a point 

 of the corona where the blackness had a certain density and, without turning the 

 microscope, made similar measurements 180 degrees from the first position. Keeping 

 the degree of blackness in my mind, I repeated the operation on the other photo- 

 graphs, and then for the other position-angles. 





 4. Reductions. 



The object of the reductions is to find (l) the mean distance from the solar limb of 

 each equal-density curve, and (2) the position of each equal-intensity curve with 

 reference to its mean circular-intensity curve. The steps are as follow : 



(a) There is a slight difference amounting to a few thousandths of an inch between 

 the diameters of the moon as obtained from negatives and from positives. On the 

 negatives the lunar diameter is 0'565 of an inch, and I reduced all the measured 

 distances to this diameter by correcting them by half the difference between this 

 figure and the diameter appertaining to each measured distance. 



(b) Ifi'i/ii'-fion of the Distances front Lunar Limb to Solar Limb. M (fig. 4) is the 

 centre of the moon, A, B and S are respectively the centres of the sun at second and 

 third contacts and / seconds after the second contact. The duration of totality is 

 about 205 seconds. The diameter of the moon is D = 0'565 of an inch, and that of 

 the sun is d = 0'540 of an inch. The angle between the second and third contacts, 

 1567 degrees, is given by the first and last photographs; it equals the angle 



VOL CCVII. A. _' s 



